[arghwhat]

They of course need the signed contract back - all parties need a copy with all necessary signatures.

My point is that you should not sneak in changes with the signature. If the contract is changed, you should notify the parties of the changes, not pretend you just signed their copy. If you do not notify them, it might be seen as an attempt to trick the other parties.

How that would play out depends on your local legal system, with options ranging from "too bad", "contract or clause invalidated or reverted", all the way to "prosecuted for fraud".

"Your should just have expected me to have changed the contract and compared it with the previous version to find all the changes!" is not an argument that would get court approval up here...