The rules actually are pretty short and again I have to marvel at the straight elegance of OD&D:
- First of all the rules don't recommend to change classes for player types other than elves. But they don't forbid it either. So the referee can decide for his campaign, nice.
- Only men may change classes.
- A prime requisite (not yet explained but you probably know what it is; otherwise wait for one of the next posts) of 16 or better is required in the class your character wants to change to.
- The attribute score must be unmodified, an interesting twist further limiting the proliferation of dual classed character types.
- Magic-users cannot become clerics and vice versa.
- Character types can change their class but once. The reasoning for this limitation is that the rules (read literally) state that elves can switch freely but don't mention that for men. So its seems logical that changing but once was intended.
- Character types can only use their class specific abilities if they abide by the restrictions of the class. Again this can be derived from the elven type for which the description states that elves can combine the benefits for both classes.
Finally I will clarify the following points:
- The new class starts at level 1 (as for elves, nothing new here).
- The best bonusses for both classes are used (there is no specific ruling but this seems logical - I don't see any logic in characters forgetting skills from adventure to adventure).