I believe the proper thing to do in this situation is for some interested
party to write an informational draft that describes how, within the
context of the OP framework, one would build something that is backwards
compatible with 2.6.2. In other words, write a draft that says:
to be compatible with 2.6.2, use RSA and IDEA. Their identifiers are <xxx>
and <yyy>.
It gets the information documented. It's Informational so it doesn't
violate the IETF guidelines. It can be referenced by the main document.
It even gives you a place to write a nice speech in the Security
Considerations section about how you wish people would migrate.
So IDEA as SHOULD would guarantee backward compatibility? Fine. I'd prefer
MUST, but I see the problem.
Alas, it doesn't. The only thing that guarantees interoperability is MUST,
and you can't have a MUST algorithm that's got intellectual property
constraints, if there is an alternative.