Sounds to me like the project sponsor was just trying it on and see if you would accept that.  Could be that it would push project budget over etc.  Not the first time I have seen it and even had it happen to me (ended up leaving when we could not agree and getting a payout based on my old contract, they wanted me to sign a new contract which would have a major reducing of payout owned to me).

Some companies only care about the budget and $$'s if they push, push back and be ready to leave.

In my case a lot of other people signed the new contract, ended up getting restructured out 6 months later with a major reduction in payout due to the new contract.

I am glad yours did work out however :)