TUPE Bonus Retention Scheme: Is this a new contract or "improvement" of contract terms?

Project is being TUPE'd, new company are offering a contract with bonus payments (tied to project milestones) as a retention scheme. All other TUPE protected terms remain the same as per the old company. 1) Is this offer an elgibile TUPE transfer or is this a new contract? i.e. if we accept the new terms are we not protected bt TUPE regulations 2) If someone was to leave the new company (after transfer) how likely is it that this person would have to pay any recieved bonus back? 3) As per item 2, would this payment be the net or gross bonus? i.e. pre or post tax 4) Is it possible (legally) to move to the new company withoit the bonus scheme? i.e. move to the new company solely on the TUPE regulations?