A third tier mag can continue changing between all other third tier mags every time it levels based off its stats and the class and ID of the character currently holding it. Pretty sure it will try to change form when ever the level is divisible by 5 if it can change form, so to turn it back to a Kama, simply level it to a level divisible by 5 on your Purplenum.
A mag also only gets new PBs until it has 3. once it has its 3 PBs, it won't gain any new ones, nor will it exchange one of the PBs it has for a new one.
Edit: haha posted it right as Fyrewolf's response popped up on my end. Sorry for redundant answer.