Upgrade to 3.2 mostly worked with a read-only DB. The git repo related parts went through just fine, just as the NoteDB conversion did.
The only part where the upgrade threw errors, was when it tried to update the schema version. But that's expected on a read-only database. Hence, asking for a read-write database in T254516
The upgrade itself went well.
But it was unnecessarily slow.
For the last part it failed to take advantage of available CPUs due to the way the reindexing was implemented.
I've implemented a fix for that at https://gerrit-review.googlesource.com/c/gerrit/+/271695