Can I make my insurance choice lower than my firm?

I applied to Exeter and received an offer of A*AA rather than the AAA/AAB offer I was hoping to get, as they said they make their offers based on predicted grades? However, if I make it my firm choice, they said the offer would be reduced to AAB.
I was also debating Durham (A*AA if I do well in the entrance exam, otherwise A*A*A), and have applied to Oxford. If I put Durham as my firm however, then I would need to put a uni with lower entry requirements as my insurance choice, and so wouldn't be able to put Exeter as the offer for Exeter as an insurance would be A*AA.
If I put Durham as an insurance, would I be able to decline Exeter and go there if I achieved their offer? Not sure what do, as I don't want to risk firming Durham and not making the grades, as I won't be able to make Exeter my insurance.