We learned alot about upgrading during the M1->M2 upgrade - currently, the upgrade functionality (what was the upgradeM1M2.pl script) is built into the install.
So - the upgrade process will be, run install.sh (or the mac installer), run zmsetup.pl, which will figure out where you are, and do whatever needs to be done to make your new system right.
We've built it with an eye towards robustness, and also towards data integrity - so the upgrade will (99%) work - but we won't toast your data, no matter what...
(Of course, you're all backing up your mail systems, right? Of course you are

)