I apologize for the newbie question, but I've not been able to get a straightforward answer to this question... I understand you can purchase a complete Whisper or Blackout upper, but that's not what I'm asking. Is it possible to simply replace the barrel in a Sport II with a 10.5" 300 AAC barrel (pistol length gas tube), add suppressor (assuming both SBR and suppressor are approved by BATFE) and hand guard to create a new rifle? Are there any mechanical/physical limitations or challenges?

Should not be any mechanical/physical limitations/challenges if you have the proper tools and a little bit of knowledge. You just need to make sure that you got your SBR and suppressor paperwork taken care of, other than that everything else is the same except for the barrel and gas tube, depending on the length of the gas system you are using.

The paper work he speaks of tends to take about 6 months to clear.

It"s up to you but I would rather just get a complete upper and have two firearms. Plus, head space can be a problem sometimes unless you get a barrel and a BCG as a matching set.

I am with sgtsandman, a complete upper just seems better/simpler to me than deconstructing a perfectly good upper.

I appreciate the feedback. My chief concern is the issue raised by sgtsandman regarding headspace. I think that's probably one of the main reasons for considering a complete upper. Thanks again for the insights.