As I said above, the only supported upgrade path that I am currently planning to support is creating a new 18.04 machine and then restoring data from a backup.
(Also, the Ubuntu 14.04 box will need to be upgraded to Mail-in-a-Box v0.29 before doing the backup/restore.)
The Ubuntu 18.04 branch is almost usable, but I’m having a serious problem with backups so I’m not recommending using it just yet.