Gyms suck that way, thats why you never prepay your memberships, especially when they're running some "huge special" if you buy two years in advance that is waaaay too good to be true.

Legally, atleast in the states, if they have another gym they can get to agree to honor their paid in full memberships not their EFT monthly ones they are ok with no liability. The gym has to be of equal quality though and the distance has to be around the same (cant remember the exact distance).