If the first contract was a binding contract and not just outlining the employment terms for at-will employment, then you have a case that requiring the second amended agreement was a breach of the first one.

More likely though the first one was not a contract (those are rare and the presumption is against them). The terms of at-will employment (including salary terms, bonus terms and conditions, etc.) can generally be changed by an employer whenever, on the theory that they could fire you anyway for any reason and so why would there be a problem with them telling you they will fire you if you don't accept a pay change?