I'm replacing the A1 upper on my pre-ban 14.5" with a flattop. Do I need to keep the old upper after the work is done. I've had offers to buy the old one but not sure if I need to keep all the old parts to stay legal.

You only need to keep around enough parts to build the rifle into a pre-ban config - not the exact original parts that came with it.

If your new flat-top upper is in post-ban config, you'll need to make sure you've got a pre-ban config upper or an evil telestock somewhere around.  If the new upper's already a pre-ban config, no worries.

Keeping parts around to assemble the AW is only necessary for maintaining its grandfathered status if it's not a Colt SP1 or a Colt AR15-A2 Sporter II, or a Colt AR-15A3 Tactical Carbine, since those rifles are preban by name and not by features count.

I should have been more specific in my first post. I am replacing the upper [u]RECEIVER[/u] with a flattop. I am keeping the same barrel on the gun. (Funny the difference one word makes! [B)])

Eventually I may get another barrel and entire upper assembly to switch off with the original barrel. I know that I'll have to keep the short barrel around.