The only Black & Decker router that you can change the collet size on is the FS1200RP; all other Black & Decker routers are strictly 1/4" or 1/2" units.  The FS1200RP can be adapted to either a 1/4" or 1/2" collet sized unit.

The FS1200RP has a 1/2" collet on it, and it's possible to fit the enclosed 1/4" collet inside the 1/2" collet.  In order to remove the 1/4" collet from inside the 1/2" collet, you might have to use a pair of pliers.