You can do an automatic upgrade thing, where you recognise old formats and upgrade them on the fly. Possibly with a window on it where at sufficient age people need to run their elderly code through multiple upgrade cycles.

Or you can declare it an unstable interface and it's on the user to deal with it changing over time.

Or you can just leave it accessible without excessive work, but not document it, and then it's very obviously on the external tool to deal with the impedance matching.