I just went to Porter Cable's site, and looking at the pictures, it seems like two routers (7518 and 7519) can at least be removed from their fixed bases. Unfortunately, the optional plunge base (6931) doesn't list either of those two routers as being compatible. I would assume that they are not compatible, but you could try calling PC. All the other 3.25 routers appear from their pictures to have bases as an integral part of the casing, and don't look like they can be removed.

It looks like PC only has the smaller 1.75 HP routers with switchable bases.