Is there a way to save members in a different database? (I'm talking about the whole member's tables not member properties)
The reason for me asking this is to not worry about members data during content freeze implementation and parallel website migration.
You can easily make your own implementation of a MembershipProvider and replace this with the build-in membership provider in the web.config.
However, I would rather review your website upgrade process to avoid a "content" freeze. E.g. if you use the Umbraco Cloud offering, you can push your changes online to upgrade a website while maintaining your content).
Also packages like Umbraco courrier, Umbraco Deploy, chauffeur and USync allow you to rule out a "content freeze".