Me and my terrible memory, I'd need to look at Bruno's code for the new FAC to be sure, but I suspect it wouldn't take more than an hour to come up with a workable SQL Query to recalculate everyone's FAC. It's then just a case of giving the query to MySQL and waiting for it to finish recalculating.
EDIT: On the assumption that the new FAC algorithm hasn't changed from Bruno's original post, there's nothing in it that would cause too much difficulty - from a development standpoint, all the data is easy enough to get at in SQL (a few LEFT JOINs for sure). I'm almost tempted to devote a bit of time to it tonight and see what my own PP FAC is under the new algorithm.