What about Copa?
 

I wonder if they will still be closely tied with the new United. I suppose they could switch to the new Mileage Plus.

No reason to think they wouldn't be, although CO divested most of their ownership stake in Copa a while back. I imagine they'll either adopt M+ as their FF program or team up with AeroRepublica to create one of their own.

do we know for sure that the PC and OnePass brands that CM uses will disappear?

My guess is that they'll join Mileage +

We don't know it won't remain OnePass now...United OnePass...

United has been friendly with TACA recently with code shares, M+ earning, and such. I think Copa must be in a strange position regarding with whom they will run their FF program. They may feel about as confused as I do about what program to use.

TACA also submitted an application to Star Alliance. TACA and Copa are extremely similar and I just can't see *A taking both TACA and Copa. TACA's alliance with United is not nearly as close as Continental's with Copa.

For this reason, I see Copa eventually joining *A and the new UA FFP. TACA and their new friend Avianca will in my view eventually go to Skyteam as Avianca and DL have been partners from some time and Skyteam is in desparate need of a S. American member.

Star Alliance can absolutely take both TACA and COPA. There's no good reason for it not to if both apply and meet joining requirements.

They don't even need to repaint their aircraft. Unless they want to rename to Uapa, Unpa, Ucpa, or something like that.