[gspencley]

IANAL but it seems to me that this "law" already exists.

A TOS is a contract. It literally stands for "Terms of Service." Meaning, you give me money and here are the terms under which I will offer you the service you are paying for. How enforceable that "contract" is depends on a ton of things, differing in various jurisdictions (law is complicated), but it is - at the end of the day - a contract.

So I don't know how actionable it is, but the OP said that the company considered changing their TOS for currently active users. That could, in theory, be breach of contract and the customers might have a claim (again IANAL).

[There could have also been a clause in the TOS saying that they could change the terms at any time for any reason - though I suspect in many if not most jurisdictions, that would make the entire contract unenforceable].

In your case, don't make [potentially] contractually binding promises that you can't or don't want to keep.