Funny story last year I got offered a Distinction from my firm university, after I firmed them I get a phone call saying my grade offering of a Distinction was a 'mistake' and they changed it to a lower offering of Merit. How much of a 'mistake' this was I'm not sure, but I had already firmed them so I'm not sure what their intention was. All my offers were jacked up compared to everyone else probably because on my reference I was stated as predicted to achieve Distinction.
Despite them offering me Merit, I still got Distinction anyway!
Just wanted to put that out there to say offers can be changed even after the university has been firmed or insured. At least I've heard of situations of offers being lowered such as mine, but not increased.
Likely your offer won't change, you got an offer of ABB, that's their general entry requirement and probably what you'll get. I wouldn't worry at all.