I think the point about the conversion scale with subgrades makes a lot of sense - because of the introduction of the new "Mint+" (new 9.5) grade, there were old green-label 9's that surely would have met the Mint+ (new 9.5) scale. Whilst there is no perfect solution - at least, for the customers who have submitted with subgrades previously, there is a feasible path forward with available information.
Something like:
If total subgrades of the old label are less than 0.5 off from qualifying for an old 9.5 (Gem-Mint) (for example, 2x 9.5, 2x 9.0) - then these should automatically be mapped to the new 9.5 "Mint+". Aside from that, they will be map from an old 9 to a new 9.
At least, this seems fair to customers whom have paid for the service of subgrades previously.