Legal Question in Business Law in California

Ordinarily, the original contract remains in effect, and the altered version is of no effect or consequence.

Exceptions are possible, but unlikely and rare. The alteration might void the contract. Or, the alteration might be valid and enforceable. But ordinarily, altering a contract after it is signed by the parties has no more effect than spilling coffee on it; i.e., none.

If you send me particuars directly, maybe I can give you more specific advice as to whether this alteration might fit either exception.

No, because there is no meeting of the minds. The parties agreed to the original terms, not terms unilaterally altered by one party. It would only be enforceable if both parties agreed to the new, altered terms.

It depends on when the alteration is made and what happens after the alteration. You have received simplistic answers to what may or may not be a simple situation. It depends on the facts. For example if it is a UCC contract you can have a classic UCC "battle of the forms" where one side sends a quote, the other responds with a purchase order, and then the other responds with a sales order or invoice, each insisting that their own terms and conditions control. Or one might make an alteration on the form from the other and send it back. Even outside the scope of the UCC, if one person sends a signed contract, the other person alters and signs it and sends it back and the first person receives it without objection and begins performing the contract, that may make the revision effective. You have to look at the actual facts and documents, and history of the transaction, to determine if that alteration is binding or not.