I'm only offering a data point here, and not discounting the troubles mentioned above. Yesterday I got the upgrade email early in the morning for a flight next week. I quickly selected a seat without issue.

But I agree that Copa's IT systems are inadequate.

Hello! It's already 2020 and sad to say, we're still having this same problem. I just want to ask, were you able to eventually get your itinerary sent to your email? Were you able to successfully fly out? Thanks in advance.

Same issue
 

It's 2024 and the problem persists. Me and my parents bought bussiness class tickets and while they managed to secure their seats mine says its unavaible and they're all greyed out. Called COPA a few times now and they say they are looking into it but nothing so far.