Can I replace a fan with a lower-rated fan?
Would a 12VDC fan work if the dimensions were the same?


Unlikely. Supplying a fan with a higher voltage than it was designed for will usually cause it to run faster than intended, and produce more heat. Both of these will significantly shorten its lifetime.

12V to 14-15V might be close enough that the fan will tolerate it, but I wouldn't want to risk it.