There's a whole lot of cases where the tokens are temporary in nature with an easy cut-over, either dropping old entries or re-encrypting while people are not at work. We tend to think of big commerce like amazon or google that need 24/7 uptime, but most individual systems are not of that scale

In most other cases you increment the version number for the new data format and copy-paste the (d)e(n)cryption code for each branch of the if statement, substituting 128 for 256. That's still a trivial change to substitute one algorithm for another

Only if there exists no upgrade path in the first place, you have a big problem upgrading the rest of your cryptography anyway and here it's worth evaluating per-case whether the situation is considered vulnerable before doing a backwards-incompatible change. Just like how people are (still) dealing with md5