What brand of fan is it? And are you trying to upgrade the fan to a bigger CFM or just rip it out and replace it?
They make upgrade kits for many types of fans where all you do is swap motors, but if it is too old then maybe not.
Can you get into the roof/ rafters? you should be able to. I am imagining that there are some tabs that are bent over onthe other side or that they may have put a board or glue in place to hold it up better.
[CENTER]I am sorry, but "flippers" drive me nuts; Just because some girl in Cali did it on this one show and made 100K, doesn't mean you can do a half-job and turn a profit[/CENTER]