This can potentially corrupt your game save make sure to copy or backup your save game folders

There are two versions of GRW. The stand alone that does not use Steam but does use the Uplay launcher and the Steam version which uses a cut down Uplay launcher to launch the game and deal with the Uplay cloud saves.

You maybe aware Uplay games on Steam don't normally use Valve’s cloud but use their own Uplay cloud to save your games progress. They also store a copy locally in the Ubisoft folder in your Programs Folder.

Example C:\Program Files (x86)\Ubisoft\Ubisoft Game Launcher\savegames\

In this folder should be a folder which has all your Uplay game saves. The problem I had was the UplayPC and SteamPC versions save the game to two separate folders Uplay 1771 and Steam 3559.

• If you're coming from Uplay and want your progression carried over then rename the Steam PC save game folder from 3559 to Steam3559 and then copy and/or rename the UplayPC save game folder from 1771 to 3559
  
  
• Next time you run the game from steam it should ask which save you want to use the local or the cloud. Choose local you should now have your free weekend progress from your Uplay GRW on your Steam GRW.
  
  
• If you play on Uplay after you renamed the 1771 saves folder Uplay should recreate the folder from the Uplay cloud.

SAVE GAME MANAGEMENT @Uplay[support.ubi.com]

This is where i got the game code numbers for the UplayPC and SteamPC versions and it’s also tells you what to do if it all goes wrong