Vulnerability in EPiServer.Forms
How to migrate users/groups from CMS 11 to CMS 12?
What membership system does your CMS 11 site use?
Legacy asp.net membership (SQL Provider) or something fresher?
Checkout the blog post here https://world.optimizely.com/blogs/K-Khan-/Dates/2017/10/migrate-from-sql-membership-to-asp-net-identity/.
Should help with your migration.
The following blog post has detailed steps what you should do to upgrade to CMS 12 Core Identity migration, thanks for Geta's author :)