Well, it sorta depends which type of FSA compact cranks you have... there are some with the semi-integrated MegaExo BB, and others that use the ISIS style.
If you have the integrated style BB, it'd be a huge PITA since you'd have to completely remove the BB, put a standard BB style in, and then attach the Powercranks, and make sure to get everything back in adjustment again. And then do the reverse to switch back. Can't really give you a more specific answer, since the tools involved depend on what sort of replacement BB you choose, in addition to the special spanner wrench for the MegaExo.
If you have the ISIS type, you've lucked out and all you need is an allen wrench to remove the bolts, pull the cranks off, put the new ones on and put the bolts back in. (That is, of course, assuming that you buy the Powercranks with the right style to match the existing ISIS BB.)