(See the best response by Kaiochao.)
I'm trying to be able to change a movement key from NORTH+REP to NORTH+UP with winset, but can't figure out how to. I'm sure this is a super stupid question but I can't get it to work.

I tried this, didn't work.
winset(src, "macro.northkey", "key=NORTH+UP")

The intent is to be able to change movement from holding the key down to only when it's released with winset.

The ID of the north movement key is northkey

Best response
The parameter for the key combination is "name", not "key":
winset(src, "macro.northkey", "name=North+Up"
Ahhh, duh. Thanks!