Does a new contract that supersedes all other contracts trump an irrevocabled clause in a contract in effect at the time of breach?

There is no such thing as an irrevocable clause, but also no such thing as a contract which allows one party unilaterally to modify it (such a contract would be considered illusory).  You agreed to revoke the clause.  The question is whether your agreement to the modification is void because it was made under duress, and legally I think you have a very tough case.  Your claim would not be based on physical duress (nobody held a literal gun to your head) but rather what is known as economic duress.  I'm not sure whether the clause on which you're relying also prevented you from ever being fired, or ever being fired without cause.  If not, there is nothing wrong with saying that you'lll be fired if you don't accept the new contract.  Even if you couldn't be fired under your old contract, generally you can't void a contract for economic duress if you could have gone to court to avoid the duress in the first place, which seems to be the case here.  I think this claim is weak, but I wonder if you can sustain a claim based on age discrimination?  I don't know the underlying facts about whether younger employees have been treated better, but this might be an avenue to pursue.