On Thursday, developers enabled the pre-download of the updated PC version of GTA 5. It can be found under the name Grand Theft Auto V Enhanced and can be downloaded from the game library in the Rockstar Games Launcher (91.53 GB) or on Steam (91.69 GB). As of now, it has not appeared in the Epic Games Store. It might be available there on the release day of the upgrade — Tuesday, March 4th.
In story mode, you can already transfer a save from Legacy to Enhanced. This can only be done between PC versions and with only one save file. Further progress will not be synced and cannot be transferred back to the old version.
Source: gta.com.ua