Currently, we use a "removable" mag block that inserts into the magwell from the bottom. The mag catch is used to locate it, and a set screw in the bottom of the block's body pushes on two ball bearings, which in turn force two retaining pins out from the block's body into the interior wall of the magwell, retaining it in place. While it would be removable if desired to switch over to a different caliber upper, it really isn't that practical.
The 9mm conversion uses a different hammer (that can be used with 223, but a 223 hammer doesn't work w/9mm) that wouldn't necssarily need to be removed. The receiver extension (buffer tube) length that you're using determines which buffer set up you'll have. CAR length just uses a CAR action spring and a heavy 9mm CAR buffer, that could also be used for 223. If you're going to use a rifle length extension, then for 9mm you use a stock spacer, CAR action spring, and 9mm heavy CAR buffer, which would have to be swapped out to a rifle spring and buffer before transitioning to a rifle caliber upper. The big glitch goes back to the mag block. Once in and adjusted properly to feed, most people I know don't want to take it out again.
So, while the conversion is reversible in fairly short order, it is kind of a pain to go back and forth.