T. Foster said:
FWIW OD&D doesn't explicitly limit class-changing to humans, it's just that since the only non-human race with more than one choice of class in the boxed set is elves, and they all switch classes automatically, it's de facto a human-only option. But, once the thief class is added in Supplement I, if we assume the same class-changing rules still apply (i.e. must have a score of 16+ in the prime requisite of the new class; clerics can't become magic-users or vice versa) there's nothing really to prohibit, say, an enterprising hobbit with Dex 16+ from working up to 4th level as fighter and then switching over to become a thief...