We have adopted the Class-Changing system from the Net AD&D Players Supplement by Dave Medvitz. It allows for flexible class changing based on adventure experience. With good GM control, this system allows PCs to change classes with each experience point award instead of being roped into a single or multi-class situation for the life of the character. We find that this system promotes character longevity and creativity.
A character can only switch classes immediately after advancing a level. *All* experience henceforth goes toward learning then advancing in the new class. However, the character does not yet have any of the abilities of the new class. He/she must first gain experience sufficient to advance to second level in the new class for Rogue and Warrior classes, and sufficient to reach third level for Priest and Wizard classes, before gaining the abilities of a first-level member of the class. After gaining the requisite experience, the character becomes a first-level member of the class in question. (Experience gained while training is not applied to the new class, it is assumed to simulate the time necessary to learn a new way of doing things--in other words, it "goes bye-bye".) The 10% experience bonus for high prime requisites does apply--consider the character to be taking advantage of natural talent for the area to advance quickly.
Characters changing between subclasses receive a 20% bonus to any experience gained at the "pre-first" stage of the new class. Characters changing between different special backgrounds (from the Complete Fighter, Cleric, etc. manuals) within the same class receive a 30% xp bonus at the "pre-first" stage, but must have explicit permission from the GM. (Require a damn damn damn good justification for switching/gaining special backgrounds.) These bonuses are not cumulative with each other. This simulates the fact that such characters already know much of the basics in these areas. The high prime requisite bonus is halved (5% bonus) for the "pre-first" stage in such cases.
When attaining the first level in a new class, the character has all the abilities of the new class save one: She does not gain as many proficiencies as would a character starting in such a class. She only gains half (round *down*) the proficiencies that her new class starts with. Assume that the "missing" proficiencies are normally learned in childhood or pre-apprenticeship, hence the character hasn't the opportunity to learn so many in such a short time. GMs who feel this gives too much can use a different fraction.
Note that the character is fully capable of using all her abilities at any time, subject to the limitations of each class. In other words, a former warrior turned mage *DOES NOT FORGET HOW TO USE A SWORD*. However, Wizards can still wear no armor other than enchanted Elven Chain if they wish to cast spells, etc.
The character uses the highest of his/her class levels to determine attack and defense bonus.
UNDER NO CIRCUMSTANCES CAN A CHARACTER ALLOCATE EXPERIENCE TO MORE THAN ONE CLASS AT A TIME, NOR MAY HE/SHE "SPLIT" EXPERIENCE AMONG THEM. CHARACTERS ADVANCE IN ONE, AND ONLY ONE CLASS AT A TIME. However, characters may change which class they allocate experience to whenever they gain a level. Furthermore, if a character makes the attribute requirements for changing classes, characters may change classes as often as they like.
"Multi-Class" Characters, therefore, do not exist under this system. In other words, no character, no matter what she is, may advance in more than one class *AT A TIME*. Instead, characters may change class more freely than in "official" AD&D (see below). Note that this system will tend to annoy power- gaming players since it will cancel one of the "advantages" of being a demi- human. However, It will allow the GM to do away with level limitations without amateur demographers complaining about the "illogic" of humans ruling the world.
GMs may wish to place attribute requirements upon characters who wish to change classes. Three levels of conservativeness are given below.
While I state that "all abilities are gained", I recommend either eliminating weapons specialization bonuses for characters with more than one class or toggling them--ruling that the character only has time to keep her skills at peak perfection while he/she is progressing as a warrior. At other times, her fighting ability deteriorates since he/she isn't practicing enough.
Note that the specialization bonuses might not be toggled or dropped for a character who is playing a Warrior Priest (special background). This is up to GM decision.
For you GMs who look at this system and scream "Power-gamer! Power-gamer! Nyah!" I offer the following:
Many players would not even take this, the most efficient, route, considering how low a survivability MUs have, even in the revised combat system. (I recommend you delete the preceding paragraph before using this with players--it might give them bad ideas.)