I'm not convinced. If one enters into a contract for some work, agreeing to pay a certain amount on completion, and then when the work was substantially done, tried to enforce a change to a lower price, I think we'd all agree that was an attempt to retroactively change the contract.

I'm not sure how changing the rules on redundancy payouts shortly before making lots of people redundant is very different.

(On the other hand, I also have limited sympathy with the idea that capping the payout at a year's salary is a terrible hardship. I'm concerned that there may be other changes other than the headline one which means significantly reduced payments for people who would be getting relatively small ones in any case. Haven't done the research though.)