I have had a few re-ticketing issues when UUAing, although in MMB it shows as confirmed in the new class when I tried to select seats in advance it wouldn't let me saying that it hasn't yet been ticketed. I could also see that my card hadn't been charged (although the points always disappear straight away) for the change in taxes. When I looked at the flight in my BA app I could see there was no ticket number there either. I called and they said I need not worry I was confirmed and they ticketing team re-ticket on a priority basis so as my flights weren't for a few months I am just in a queue, surely enough after a few more days I got an automated email that I had changed the booking and it had been re-ticketed (and my card charged).
Interestingly on one occasion over the phone I upgraded from WT to WT+ using cash and then UUAed to CW (it was a amazing good price and I couldn't believe it). I didn't get re-ticketed for ages and out of the blue had a voicemail on my mobile saying I had to call them regarding an upgrade on my upcoming JFK flight. It was a Friday night and past 8pm before I realised I had the message so I just left it until Monday morning and luckily I did because when I called on Monday they said I had been charged the wrong price, it should have been quadruple but out of good will because they couldn't get hold of me they re ticketed it anyways and met the costs internally
