My thoughts on LotFP multiclassing: Retraining

One of my favorite things about LotFP is the clear-cut delineation between the classes. The fighter hit things; other characters can try to hit things, but they kinda suck at it. The specialist does adventuring things; other characters can try their hand at skill checks, but they'll likely fail, plus only the specialist is able to perform sneak attacks. The magic-user gets to use magic, and that's pretty much all they do. However, a lot of my players are coming from 3.5 or 5e, and they enjoy the freedom of multiclassing. Who am I to stand in the way of them making meaningful decisions about their characters? So I'm going to start allowing class-switching (with some restrictions) as a house rule and see where it takes me.

Note: for the purposes of this post, I'm going to ignore clerics because Raggi is banishing them to the appendix, but everything I talk about applies to them as well. I'm also ignoring the demihumans for the same reason, and because they don't even slightly fit in to these rules.

LotFP Retraining Variant Rule:

So you want to change your class?

The first thing to understand about these rules is that they are not 3.5-style multiclassing. This is Retraining: unlearning unhelpful/bad habits and learning how to do the things your new class does. It is an involved process and is necessarily restrictive. One doesn't just magically become proficient at, well, magic. It takes time and resources to learn an entirely new approach to adventuring, and there are many steps along the way.

1. Before Retraining can occur, the adventurer must find a person who is both willing and able to instruct them in his/her newly chosen profession. We will first deal with ability. 
• Fighters are easy to come by, generally the people able to train an adventurer are captains in a town/city guard, a gladiator or other fighting performer, a skilled veteran, etc. They can be found almost anywhere, but are easier to come by in cities. There is a 3 in 6 chance of finding an able instructor in a town, and a 4 in 6 chance per day of searching of finding one in a city (limited by the population of the city, 1 attempt per every 1,000 people).
• Specialists are a rarer breed. People with these skills are often adventurers themselves, and those able to train another are usually successful at that. There is a 1 in 6 chance of locating one in a town, and a 1 in 6 chance per day of searching of finding one in a city (limited to one attempt per 1,000 people living there).
• Magic-Users are a reclusive bunch, what with the more "civilized" populace constantly trying to burn them at the stake for witchcraft. Locating a magic-user trainer can be an adventure in and of itself. There is no chance whatsoever of locating one in a city or town, because even if one was actually present, s/he would likely be concealing his/her profession. Even so, their presences far outside of civilized lands are often known, and getting this information is much like any other adventure hook. The referee should determine if any suitable magic-users are in the area when a player decides s/he wants to Retrain to this class. A magic-user must have at least as many levels as the person being retrained does of their initial class. So a 10th-level fighter would need to locate a 10th-level magic-user to retrain.
2. Once a suitable trainer is located, they must be convinced to train the character. Often, this is as easy as a rather sizable payment (for reasons that will soon become clear). Other times, it may require the character to perform a service for the trainer. These services can be almost anything the referee deems appropriate. A trainer who requests a service is unlikely to be swayed by an offer of money, unless that offer is unusually generous. Magic-user trainers often request services and reject any payment, but this is not universal. When accepted, payment is usually on the order of 200 sp per month of training. Additionally, any equipment that the trainee requires (weapons and armor, spellbook, tools, etc.) must be purchased separately.
3. After the trainer has been found and convinced to train the character, training can actually begin. Retraining takes two months, plus one month for each level that the character had in his/her previous class. Retraining to become a magic-user takes twice as much time. During this period, the character learns to do all the ways their new class does the things they do, and unlearns all of the unhelpful things that their old class did. For example, a fighter-to-specialist might learn how to pick locks with a delicate touch, unlearning the forceful nature of his/her previous class. The result of all this "unlearning" is that the character's old skills atrophy somewhat for lack of use.
• A fighter who undergoes Retraining at level 2 or higher loses one point of attack bonus that was gained for being a fighter (it doesn't affect the strength modifier). A 1st level fighter who Retrains retains the +2 attack bonus, but instead becomes worse at the Press and Defensive Fighting actions. These now only provide +1 to-hit, -4 AC, and +1 AC, -4 to-hit respectively (full AC bonus is retained on Defensive.Fighting if wearing a shield).
• A specialist who retrains at 1st level loses one point in any skill (player's choice) and one additional point in any skill for every two levels attained at time of Retraining. For example, a 3rd or 4th level specialist loses two points from any skill/s, but a 5th level specialist loses three points. Additionally, the maximum value any skill may have is reduced to 5 in 6. Any skill that has a 6 in 6 loses a point automatically, before the skill points lost from Retraining are assigned.
• A magic-user who retrains loses one spell slot of each spell level that has more than one spell slot. For example, a 7th level magic-user normally has three 1st level slots, three at 2nd level, two at 3rd and one at 4th. This character would then lose one slot each from 1st, 2nd and 3rd level, but not from 4th, because there's only one slot at that level. Additionally, whenever the Retrained magic-user casts a spell where the effect depends on the level of the magic-user, s/he counts as one level lower. So a 7th level magic-user only shoots six missiles with Magic Missile and Invisibility would only last six turns.
4. Upon completion of the Retraining period, the character immediately gains one level of the chosen class, and is considered 1st level for all abilities that class has.
• A character who Retrains to fighter gains a +2 attack bonus, and access to the Press and Defensive Fighting actions.
• A character who Retrains to specialist gains 4 skill points to be allocated as the player sees fit.
• A character who Retrains to Magic-User gains a single 1st level spell slot, and the spellbook s/he bought gains the Read Magic spell, along with three other randomly determined spells.
5. A character who advances this way gains hit points as normal for his/her new class.
6. If any of the saving throws for 1st level in the new class are better than any of the character's saving throws before retraining, then that saving throw is replaced with the better value. For example, a 2nd level fighter has a Save vs. Poison of 12, and a Save vs. Magic of 16, if s/he retrains as a magic-user, s/he would replace the Save vs. Magic with the 1st level magic-user value of 14, but retain the original value of 12 for the Poison Save, because it's better than the 13 that magic-users start with. This same process is performed whenever the character advances in his/her new class, only taking the new class's saving throws if they are better than his/her old class's.
7. Once a character completes Retraining, s/he advances exclusively as his/her new class. A specialist who retrains to fighter no longer gains skill points when s/he attains a new level. For the purposes of determining any effect that uses the level or hit dice of the target, such as Cloudkill, the Retrained character's level is considered to be the sum of both of his/her class levels. As such, a specialist 2/magic-user 2 (a 2nd level specialist who has Retrained as a magic-user and since attained another level) is unaffected by the spell.
8. A retrained character gains levels in the new class when s/he gain enough XP to advance as though s/he had class levels equal to his/her total levels. For example, a 3rd level magic-user has 4,500 XP and decides to Retrain as a fighter. After the Retraining period, s/he is now a magic-user 3/fighter 1 still with 4,500 XP. Since his/her total level is 4, in order to advance to magic-user 3/fighter 2, s/he will need enough XP to reach 5th level as a fighter, which is 16,000.
9. A character is still affected by any limitations that his/her original class suffered from. A magic-user who Retrains as a fighter does not gain the ability to cast spells while wearing encumbering armor, etc.
10. Finally, a character who has already Retrained, but wishes to resume advancing as his/her original class may undergo Retraining a second time, at full price, but the time is based only on levels gained in the new class since retraining. If you retrain this way, you regain anything lost to atrophy, but your second class atrophies as normal. A character may also Retrain a second time if s/he wishes to have levels of all three classes, but this uses the full character level to determine the time it takes to Retrain. If a character Retrains in this way, both classes experience atrophy as normal.

If you use clerics in your campaign, then they should not be allowed to Retrain as magic-users, and magic-users should not be allowed to Retrain as clerics. If your campaign includes demihumans, they should not be allowed to retrain like this, though that's up to you. Either way, a human character should not be permitted under any circumstances to retrain as a demihuman (mostly because it makes no sense).