It may seem like a simple idea but would probably be pretty hard to code in and wouldnt(imo) be worth the time Plus I think If you are able to change nades you would also be allowed to changes weapons, classes, and skills... Im not sure if it can be setup for 1 item
I think that what he means is via the class selection screen, and only those grenades that your class can use, which I think is reasonable, both to program and realistically.
Maybe even just switch grenade types in the class selection screen, and the next time you use an ammo box/APC, it will automatically switch. That would probably be the easiest way to program it.