I was thinking about this one and I suspect it is actually a glitch, they can't really be intending to clear miles+copay ahead of SWU on international flights. The copay is effectively just prepaid in the case of a SWU anyway because of the fare requirements and the passenger has somewhat of a right to ask for a partial refund if miles+copay upgrade when SWU doesn't. I hope they get this fixed soon, I am afraid of using UA SWU's now.