Ask a Business Lawyer. Get Business Law Questions Answered ASAP.
An amended contract supersedes the original entirely. Consequently, depending upon the terms, you could be held to the contract you originally signed. However, if you already performed according to the original contract, then there would be no consideration for the prior performance with which to bind you to a new contract, and you could argue that the contract only applies from the date of signing.
To avoid a dispute, I would simply make the effective date of the amended contract, the date that you sign. That way, any discussion over what took place prior thereto would be moot/irrelevant.
I hope I've answered your question. Please let me know if you require further clarification. And, please provide a positive feedback rating for my answer -- otherwise, I receive nothing for my efforts in your behalf.
Thanks again for using Justanswer!