jefelon let's try to keep the conversation focused. We can talk through your issue here, but commenting on other unrelated threads will not get you an answer any faster.
When you say "without failed logins", are you asking about migrating while the site is still running? Or are you worried about (or currently experiencing) authentication failures after migrating? This distinction will lead to 2 VERY different answers.
Also, we have no way of knowing how your authentication is setup. It could easily be that your users are all stored in the "system" (non-tenant) database, so migrating "one-by-one" isn't even an option.
Finally, you mention "login failures", but is it truly an authentication failure (i.e. invalid password)? Or is it another error masquerading as an authentication failure. Providing a stacktrace from the logs would help a lot.