It was cheaper and easier to change out the barrel on the PS90 years ago than it is now. I actually had money down on a short barreled receiver like 5-6 years ago thru CMMG (figured I could just swap receivers back and forth into the stock - and, I wouldn't void the FN warranty that way either).
I paid CMMG the money for the new receiver and did the trust route. They sent their paperwork into the ATF. After 2+ months, I called to check on their status. I was told by a CMMG rep that they had accidentally sent in their expired FFL #... They had just resent everything the week before I called.
At that point, I got my money back and decided not to worry about it. It was kinda aggravating.
I'm fine with the PS90 as is. I don't like the strings tied to the weapon that has been short barreled (can't cross state lines w/o permission). I get extra velocity with the extra 6 inches of barrel too. Even with the 16" barrel, the gun is shorter than a short barreled AR15.
I have no plans to change the barrel