Open the Ubisoft Connect launcher settings and uncheck "Enable cloud save synchronization" to prevent the launcher from overwriting your new file with your old cloud data.
C:\Users\ \AppData\Roaming\Ubisoft\Assassin's Creed 3\Saved Games Assassin's Creed III Remastered Ubisoft Connect download assassin 39-s creed 3 save game sequence 4