https://wccftech.com/gta-online-ps5-xbox-series-x-save-transfer-issues-rockstar-fixes/

Grand Theft Auto V: Expanded and Enhanced and GTA Online launch on Xbox Series X/S and PS5 tomorrow (they’re already out in some parts of the world) and Rockstar promises you can transfer your Xbox One/PS4 saves to the new version of the game, although it comes with some caveats that are causing some fan concern. As noted by a Rockstar FAQ, once you transfer your save to the new-gen consoles, it will be deleted on the last-gen consoles…

You can migrate your profile through either the GTA Online Landing Page or Pause Menu once you have signed in to the Rockstar Games Social Club on your preferred console.