PS90 SBR conversion
Once you have done the NFA registration on your PS90 and made an SBR, you can switch back and fourth. However, you CANNOT switch back and fourth as a means of not registering it with with the ATF. As soon as you put that short barrel on without a tax stamp, you're going to jail(if you get caught). Now as far as swapping reciever back and fourth(like ship suggested), having a second reciever that is NFA registered is the way to go, if you want to go somewhere that SBR's are not allowed, simply put on your 16" reciever, and you're good to go. In an SBR friendly state, pop on that short reciever and make some flame, noise, and awe.
As far as unregistering it, once the reciever is an NFA item, it is that way forever, and is subject to all of the rules regarding out of state transit, transfer of posession, and posession, so there is no registering and unregistering the reciever no matter what barrel it has. The only way that will satisfy the ATF that it is no longer an NFA item is if you furnish proof that it is destroyed. Sounds drastic, but that was straight from an examiner at the NFA, and a local agent at the Boston Field Office.
Hope that is helpful.
Gregg