Well I'm not sure whether aircraft age is a red herring any more. The 767 is already described as needing 'substantial upgrade, including engine modifications' whereas the A330 needs 'minimal modification' (open source information).
The bidder who trusts that MoD won't notice capability in excess of the old requirements paper - a capability which is now clearly needed even more than ever before - is a pretty naiive bidder.
Huge penalties for re-opening the bidding process, I'm sure. The bidders' lawyers would probably eat whoever decided to try that for breakfast!
No doubt next week's morale-raising Christmas present for the forces will provide some indications as to whether the UK's strategic force-enabler will a ba hand-me-down - or a proper 21st century aeroplane.