I think I tried this some time ago but without success. I have the GOG installation of 4.04 for now, but on another machine I keep my game files on 1.32.
Could I copy my old-gen files to the same machine where I have 4.04 installed and somehow switch between playing one or the other?
I know I would have to take care about savegame files since newer ones would not be usable in the old one.
The problem that I see is that both versions are using the same path, '%userprofile%\Documents\The Witcher 3'
Would I need to interchange contents of these folders every time I switch the version of the game that I want to load?
Or are there more challenges than this one, regarding registry, other stuff?

Has anyone tried and succeeded with this?

It should work just fine. At least it did for me a few months back. Backup your Witcher3 folder in your documents and give it a try. Just be carefull with the save versions.

I tested this and it doesn't work. Witcher 1.32 is not starting, with no error message.

did u use offline installer?
it should definitely work. Also as he said , make backup and then delete everything regarding witcher 3 from my documents.
Do same procedure when switching to NG.

I have the following:
- 4.04 via GOG Galaxy,
- 1.32 version, simply copied the game's folder from another machine.

Are you suggesting that I leave the Next-gen as is and run the offline installer for 1.32?
I'm a bit fearful that it will overwrite things of the main Gog Galaxy installation / 4.04.
Is it separated somehow if you use the backup installer, are you certain?
What other places can I check, registry, others?

There is classic offline download installation in downloads.
If u are using gog galaxy its in extras tab.
Yes, instal separately

There is nothing to overwrite. Its separate folder. When installing select different folder and thats it.
Just clean my documents so it doesnt clash with config/save files.

Also by copying from another machine u might have missed some dependency libraries
Copy /paste usually works tho. If pc has all libraries installed properly upfront

You could even make a simple batch file that would rename witcher3 in my documents and auto change depending on which game u run.

What about these registry settings?
My GOG 4.04 installation has this path,
Computer\HKEY_LOCAL_MACHINE\SOFTWARE\WOW6432Node\GOG.com\Games\1495134320

Won't the offline installer also use the same GOG Game Id, 1495134320, and conflict?

Sidenote: I have one or two games installed via offline installers only and hence not present at all in GOG Galaxy, but they also each have their own registry path created in Computer\HKEY_LOCAL_MACHINE\SOFTWARE\WOW6432Node\GOG.com\Games\

So in order for this to work, I would need a different GOG Game Id I suppose? Could I use the basic Witcher 3: Wild Hunt edition, gameid = 1207664643, for this purpose, to keep it on 1.32?
At the moment, I only have the GOTY 1.31 installer, which I suppose it is then associated with already installed game, 1495134320.

So maybe if I try with the installer of the non-GOTY edition of Witcher 3, that would work? Or how do you suggest?