thats the reasin why it should be restricted to get the new job (written in the resignation letter) I don't know much about the law in the states but if you say for example "Im resigning to start as [job desc.] on [Date XY]" it should be fightable if they tend to abuse it. I would let them sign this resignation letter of course to have the proof, that they accept it.

In my case it was not really unusual because I got promoted or changed my hours. In my company (A large supermarket chain) it's usual to just get a new contract because it's easier to terminate a contract and have the new one signed immediately. But for some context our system does not offer changed or custom contracts because of regulations from our head office and the union to prevent legal troubles. So instead of writing hr to get us a change of contract it's much simplier and less time consuming than to just get a new contract done.