I worked as Ops person for a company that was database per user and that's we did. Every database had table with its current schema. On login, that current schema would be checked, if it was not up to date, RabbitMQ message was sent and database schema updater would update their database. User would get a screen saying "Updating to latest version" and it was just checking every 15 seconds to see if schema field updated. Most of time, it was done in less than 15 seconds.

For more important customers, you could always preemptively run their database schemas updates for them so they didn't see update screen.

Interesting! I use a budgeting app that has curious loading screens when you first log in. Sometimes it flies by, sometimes it takes a bit. It doesn’t appear to be related to data import, as data isn’t in sync when you log in. I wonder if what I’m seeing is a tenant migration technique like you describe.