Feature request: Downgrade from OpenEthereum 3.0.1

[quietnan]

README notes already that this does not work for 3.0+. However, because of https://github.com/openethereum/openethereum/issues/11858, many node operators will need to downgrade. We e.g. run two archiving tracing nodes and can not resync 2.5.13 from scratch. Would take 6+ months.
Thus, I think it would be important to have a way to downgrade from 3.0, and this project is the closest to that goal.

Thanks!