The value returned has changed, is this intentional?

Yes. Previously it would return False, which indicates that the upgrade has failed and needs to be aborted. Now we're saying that if we can't upgrade because it's readonly (configfile), we will just move on to the next one and let the admin deal with it.

hm, would it make sense to use exceptions then?

How do you mean this? This is not per se an exception, but rather just skipping over a specific database because it is fine that we can't update it. The instructions for dealing with non-compatible changes will be in release notes.

Well, True and False have defined meaning: upgrade failed or succeeded, but in this case it's neither, so raising an exception to catch later and keep on going to the next one sounds a little better semantically speaking.

I'm a little nitpicking here, if you do not like this approach, I'm fine with merging as is :)

Doesn't pep8 advice to use _upgrade_database(adminstore) not in [True, None]?

:thumbsup: for me, thanks for barring with my comments :)

Thanks for the review!