Since alpha 32 I think that 'tampered with' warning shouldn't be triggered. If you use MooseEdit to extract the contents of the save's lsv file, and then rename or move that file (leaving just the individual lsb/bin files and LevelCache folder), that should avoid the error.

Each save is a folder in the '..\Documents\Larian Studios\Divinity Original Sin\PlayerProfiles\<ProfileName>\Savegames' folder. Probably the easiest way to get there in Windows Explorer is under Libraries in the left column, to click on Documents, and drill down from there.