Ok...
That's all very well, but can it convert the back end databases?
If not, it's still the old tried and tested: Restore, then back up all the long retention backups, followed by just letting the short retention backups drop out of retention.
Either that, or you can just make a 'legacy' partition of your tape library to service restores from your legacy product. As the tapes drop out of retention, destroy them (or re-use, if you must) and shrink the legacy partition as appropriate.