If you want to go from Steam-GTAV to Rockstar-GTA, do the following steps:
1. Go to your Steam directory for Grand Theft Auto V (Program Files - Steam - steamapps - Common - Grand Theft Auto V)
2. Delete everything except the .rpf files and the folders. You may need to delete 'x64a.rpf' for some reason (or store it somewhere else.)
3. Download the Rockstar game client download (available via the Social Club website, click on the little gear beside your name and Game Downloads)
4. Use the .exe you just downloaded to Repair the files.
5. Launch with PlayGTAV.exe
If you want to go from Rockstar-GTA to Steam-GTAV , do the following steps:
1. Go to your game directory, wherever you saved it. I assume this will be Program Files, under Rockstar Games or Grand Theft Auto V.
2. Delete everything except the .rpf files and the folders. You may need to delete 'x64a.rpf' for some reason (or store it somewhere else.)
3. Select Grand Theft Auto V in your Steam directory and begin to install it. Then stop this installation.
4. Copy all your game files into Program Files - Steam - steamapps - Common - Grand Theft Auto V.
5. Resume installation. It shouldn't download anything more, but it may add a few more files.
6. Right click Grand Theft Auto V in your Steam games list, click Properties, click Local Files and then click Verify Integrity of Game Cache.
7. Wait for it to verify and download whatever it needs to replace (this could take a while.)