WOW.

After 15 min. of holding, got a Copa agent who told me my seat was in economy in their system, and that I would have to speak with United, as there was a "technical" problem and there's nothing they could do.

Called United, asked for *A desk, explained the problem. Rep was very confused (because it was clearly ticketed in I), and after several minutes, transferred me to a UA internet supervisor. She called Copa, and found the root of the problem, when I re-booked by seat with UA, the new ticket number never transferred to Copa's system. It then took 1.5 hours (I kid you not), for the United agent, who was very nice and patient, to get ahold of a Copa supervisor, to update the ticket number and reserve me a seat.

Thanks for the tips, glad I was persistent.