This is more of an 'info/warning' note than anything.
The release notes for 365 Backup V4 do clearly state that the upgrade process for repositories can be slow and long. This is something that was picked up by a couple of people when the V3 release happened. To their credit, Veeam have put a decent warning in the release notes for V4.
It's worth highlighting that this information is in a large 'Important Note' section at the top of page 12 of the ACTUAL release notes found here --> https://www.veeam.com/veeam_backup_micr ... tes_rn.pdf
DO NOT rely on the release notes here --> https://my.veeam.com/#/kb3035
These are too short and make no mention of repository upgrade time. They do refer you to the proper release notes but I'm sure some people will fall foul of 'TLDR' or indeed even miss the reference to the full notes by skimming (we ALL do it!).
We are running the upgrade now on our 365 repo and it has been stuck at 17% for well over 2 hours now (this is normal). With a repo around 1TB in size, we will be monitoring this closely until around midnight to see when it finishes and get a rough timescale.
May post an update on the estimated time it took once it has actually finished.
Just sharing the experience

Dom
on behalf of Geest IT