Security - Moving users to another secure site
If you have created a new site to replace a site with user logins, you can move those registered users so that they won't need to create a new login.
This option requires you to be in edit mode Advanced with Development enabled, learn more about edit modes here
Click the Security
tab on the website that you want to import users to. Click Import users
and select the source from the dropdown menu, click Import
The users will be moved from the selected site to this one.
Importing users from another site moves rather than copies them. If you make a mistake and move the users to or from the wrong source, you can move them back by using the same process.