The older Colt upper, if in good condition and complete, is worth a new upper in trade. A genuine older style Colt upper for a retro build is very sought after. The trade should minimally net you a good namebrand upper such as Rock River or Stag, complete with BUIS. If the upper you own is pristine, you may even engineer a trade for a new Colt or LMT upper. The AR15.com site will clearly indicate this and I would suggest going there for information and perhaps even for your trade. I have seen some of the industry partners make trades so they can build a retro.