I have two Bosch routers, IIRC you replace the collet and nut to go to a
smaller size. Many of the Bosch routers have self extracting collets
which are sold as a collet/nut unit. It does not easily come apart. so
you buy another unit for the size you use.
I prefer an insert as I go between 1/4" and 1/2" pretty often and
screwing the collet/nut off, and a different one on, for the change and
back again becomes tedious.