What @apoyen said is correct.
In general, also within the same major version, the following applies to live migrations: old → new version should always work. new → old can work, but there might be breaking changes.
During an upgrade, you might...
We could not use pve8 because of the old zfs version included there. We incorporated a zfs volume from another setup which only worked with the zfs version of pve9. Mount was not possible with pve8.
I would love to see some official statement on this question because absolutely everybody with a cluster runs into this while upgrading to pve9. Me I'd like to add a new pve9 node to an existing pve8 cluster ...
Migration from pve8 guest to pve9...