Does CasaOS has a way to mount local folders on containers? if that's so, the only thing you should do is first, stop Overseerr, then install Seerr, point Seerr's config folder to the same local folder as Overseerr, and run it. Seerr will migrate the configuration and everything will be as if you haven't installed anything new, Seerr looks and behaves exactly as Overseerr. Then you can delete your Overseerr installation, but make sure that process doesn't delete any local configuration.