Shanerbanner, only the gen 2 could be easily upgraded. The upgrade kit was $450 though. For gen 3 and later you will have to swap out the whole back end. The good news is that the fixed back end will fit on a TRG and you may be able to sell it to someone with a TRG who wants a fixed back end with all of the other features. It will also fit an X-ray if someone has an X-ray and wants to upgrade the buttstock. Thank you,
Justin