I have the original Witcher 3 GoTY installed from GoG/ I see on my library page the complete edition separate. I don't use GoG Galaxy.

Now the question is, does this mean I have to download the complete edition and uninstall the old one or is there and updater to convert the original version to the complete version?
Thanks.

Hi,

It's a complete new install. No upgrade patch with the offline installers.

If you are on PC yes, it should be compatible. I've already tried several of my old (GOTY and older) saves and they all worked fine. I've heard that there are problems with old saves if you are on console though.