I am considering selling my post ban dpms that has a 20 inch heavy barrel to raise funds for a pre ban rifle with a govt profile 20 inch barrel but I am concerned that I will be dissapointed with the pattern the pre ban shoots compared to my post ban rifle. can I get some feedback as to how much accuracy I can expect to lose. most of my shooting is around the 200 yd range but i'm looking to get into longer ranges

You probably won't lose that much accuracy.  You might notice a higher degree of heat shift in extended shooting strings.  Why don't you just buy a pre-ban HBAR?