ATL-STT is usually a domestic 757 with 24F. Fairly often there is a 26F version with AVOD used on one of the daily turns during winter peak season. Weekends you may see a BE configured 752 that is normally used for the tcon route. JFK-STT uses that 16F BE version on weekends.

My only guess for a downgrade would be from the 26F to 24F version, but they'd only loose 2 seats so if the op was in row 7 he'd be booted. If the flight went fro a 757 to a 752 then that is losing 10 seats at which point its pretty ugly for re-accommodating.

Without any more into from the OP its just speculation.