Platinum,
I've been on AS for about 2 years. I changed Doctors recently to a group that actively promotes AS as well as focal therapy (along with the traditional therapies). They do recommend repeat biopsy as part of the AS regiment along with MRI. If the biopsy does not change after the first few then they go to just the MRI.
So, in your case, a repeat MRI is probably a good idea, even according to medical folks who are strong believers in AS. I just had one, not much fun, but I trust what these doctors say. I'm also thinking of trying a focal therapy and the Dr. wanted up to date info to make that decision on also.
Dave