It's pretty easy to do with UA miles for a Star Alliance airline, assuming upgrade availability is open at the time you try to apply the upgrade, there are no upgrade standbys on Star Alliance upgrades.

I assume the reason you're booking full fare Y instead of P or Z (discounted business) is company policy? As full Y is going to be more expensive than P and Z in many cases.

I've never used the upgrade programs outside of Star Alliance, so can't really advise about Skyteam or OneWorld, but for *A, definitely UA is quite easy to use as it's all online. Your other option from UR is SQ miles which may or may not be a better value, check their upgrade award chart to be sure, upgrade inventory for other *A carriers is identical whether using MileagePlus or KrisFlyer miles.