If they unilaterally amend the contract to go from 6 months' notice to 12 months' notice, then presumably you'd have to give your 12 month termination notice then and there...
...and hope they don't unilaterally amend the contract in the interim to allow them to retroactively extend the termination period.
AFAIK, "unilateral amendment" should be considered at least very suspect by most courts?
Unilateral amendments appear to be fairly standard legal things:
Unilateral amendment might be a bit of a misnomer because its basically a new contract that your continued use implicitly accepts. There is never any retroactive term change. If they unilaterally change the notice period to 12 months and you reject, you would have to give your of rejection but it would be under the 6 month term because you are not accepting the new contract.
Unless there are other provisions for unilateral changes for contracts in the termination period, no new terms would apply to your final 6 months.