BT's lower offering was 40/10, it's now 52/10. TalkTalk fibre large is up to 80/20, and fibre medium is 40/2. With your line you would achieve the full sync, but it's quite a big drop in upload.
Sorry I didn't quite understand what you meant by what would TalkTalk do? They would take your money and put you on a new contract. The change in product would also trigger a DLM reset.