Bob - Are you talking about stripping it down and rebuilding it on an A3 upper?
I was going to do a "Build Your Own Upper" for the Knowledge Base, but I got lazy and I have been postponing it as my current AR build took a turn for the "smarter" give the "O"pocalypse that is coming.
I can help walk you through it, as can junho and slowrdy, along with several other members.
There are downloads you can get directly from Brownell's for free.
Here is a source
There are several on line, some even have video how to guides, but I can't link them from work. Do a google search for "How to assemble an AR-15 upper" and take a look at the links.
Let me know if you have any problems. It's pretty straightforward, but the first time is always a bit daunting until you see how easy it really is.