I just flew MAD-MIA on 12/10/17 on a ticket in IB premium economy purchased directly through BA's website. I asked at check-in at Barajas about buying an upgrade and was offered one for 300 euros. Needless to say I took it. I was not reticketed - I got a receipt for a class upgrade. My boarding pass still had my BA ticket number, although the fare class was now U (upgrade) and not T (premium economy).