Basic and probably stupid question to many of you out there in AR land.

Do I need to purchase a new bolt carrier assembly and bolt with a complete upper? In other words can I use the same BCA and bolt from my current M4 upper with the new "no ban" upper I plan to buy? This would obviously save me some expense. I have a feeling there will be potential problems this.

Thanks for the feedback.

People have done this with no problem and 99 times out of 100 it works fine. The issue is headspacing (do a search). I would rather not take chances when dealing with things that explode and and cause high pressures. For piece of mind the least I would do would be get a headspace guage and make sure your old bolt headspaces correctly to your new upper.

when you buy a headspace gage, get the "field gage", i have several NIBs that will lock up on a "NO-GO"..., which i have been told is rather common !!

The issue is wearing of the lugs.  If you want the bolt and barrel extension to wear together (safer) then keep a bolt matched with the barrel and get a new bolt for the other upper.

However, the bolt carrier can be used by both since it does not affect the headspacing.  So you can save like $40-50 on the carrier part.