After re-writing the paladin so that it's just a fighter with minor bonus clerical powers and suggesting that extraordinary monk abilities could be handled separate from class abilities, I started wondering if changing classes could be handled a little differently, so that it becomes more like picking up extra class abilities. The original rules state that you can change class if your prime ability score for the new class is 16+. It doesn't place any other restrictions on this, the way AD&D did; an AD&D Magic-User who wants to become a Fighter can't use magic while trying to earn experience as a Fighter. Hit dice get a little weird, especially when comparing to multi-class elves.
What I'm thinking is that characters who decide to add the ability of another class can keep their current cumulative experience and level, but start at an effective level of 1 in the new class. When characters earn enough experience for their next level in their starting class, their level goes up for all classes. However, such characters must still meet the minimum requirement of 16+, and only earn bonus experience based on the lowest prime ability score of all their classes.
Characters must pick up these new class abilities through adventure. Fighters who seek out a wizard to learn about magic only get a book of 1st level spells, and they can only learn spells of a new level one book at a time, and only when their level increases. Magic-Users who seek out combat training don't get the higher Fighter hit dice and can't cast spells while wearing armor, but they are otherwise uninhibited. Clerics can't become Magic-Users or vice versa, but either could opt to become Thieves.