Yeah! I'll be rich!
No, I meant "guarantee" in the software engineering sense, rather than the commercial sense. It might be that the Discourse software is designed so that there is a format for backed-up data which is deliberately kept stable across Discourse software versions; that the backup function guarantees that what it writes will conform to this format, across versions; and that the restore function guarantees that if data is in this format, it will be restored correctly, across versions.
Or it might be that each version of Discourse software has a restore function that guarantees it can correctly restore data only from that same version of backup function, with no guarantees across versions.