Published App in Open Beta testing (1000 capped) now want to move to Closed Beta

Android > Google playstore > Open to Closed beta migration.

I have "X" number of users in Open Beta testing (Anyone with access to the link can installed) with no of users capped at 1000. Now I want to move to Closed Beta Testing, wherein I will add those email address and they have to become a tester to use/install the App. How should I proceed ? Ways and options pls. Adding my view: If I move to Closed Beta, Google says that all the existing users wont have access to the installed app, but then If I add those email address again after moving to Closed Beta, will it solve my purpose, of keeping the users intact.

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