If you are playing the PlayStation 4 version of the game, you will be able to select one save file to upload to the cloud. Once you launch the PlayStation 5 version, you will then be able to download it. This method allows you to transfer between different devices, doesn’t require a PlayStation Plus subscription and only one save file is supported.
If you are playing on a PlayStation 5 console via backwards compatibility, once you download the next-gen version of the game, your PlayStation 5 will automatically read your save files.
Altenatively, you can simply use the cross progression process introduced in the update to carry over your saves.