Im a Premier Exec UA flyer, but for a vacation I want to fly US for the schedule -- and I can get MP miles thru US.

My question is - what's the easiest way to upgrade that segment? Do I need to xfer my Amex points to USAir to get an upgrade, or can I use MP miles? Advice appreciated.

You can't use UA miles or UA certs...just US miles, or US 500-miler certs. You could convert AmEx points or buy 500-miler certs, depending on what works for your situation.