I’m thinking you should wait until the fall release.
Currently, you must recreate the VMs using your existing ZVOLs. Whether a migration is coming in a future release remains to be seen. I think the Linux VM will be easier to manually migrate. Windows is another story. You should review this guide before upgrading to Fangtooth so you have an idea of what’s involved for migrating Windows.
Another thing to consider is that ZVOLs must now be imported (no longer used in place) to use Incus VMs. So your current replication tasks will no longer work. Backup methods for Incus VMs are still a WIP, and I don’t think they will be available in the UI until the fall release.
If you were going to attempt upgrading to Fangtooth, be sure to choose the clone option when importing your ZVOLs. This will at least give you some chance of reverting to EE if things go wrong.