I have a post ban Colt A2 HBAR. I want to free float and make it a flat top, but use original barrel and all. Will I have to reheadspace after making the upper receiver change. Thanks 2200v

No. Headspace is determined by the bolt lugs locking into the barrel extension lugs. If you use the same barrel and bolt, nothing has/can change.

Even if you did headspace it, there is nothing you could do about it if it was incorrect except change the bolt or barrel.

The barrel extension is screwed onto the barrel at 175lbs.  When you pull the barrel out, the barrel and the extension are going to come out/ and go back in the upper receiver barrel extension socket as one unit.  The barrel nut on the upper receiver just holds the barrel nut/extension on the upper (barrel nut locks/retains a lip on the barrel extension).

The only pisser/problem that your going to have is that you have to remove the front sight tower on the barrel to pull the delta ring (parts), then the float tube nut locks the stripped barrel/extension back onto the upper receiver.