Allright took me a while to make time to get back to this - Lisias' post, although slightly cryptic, solved it for me.
For anyone else who is also not familiar with this - you can download those manifests through steam, here's how to do it:
From steamdb.info get appID, depotID and manifestID for the thing you want
Open steam console by issuing the command through your browser
steam://nav/console
Had to do this in Firefox or edge/iexplore, Chrome did not allow for it.
Use the following console command:
download_depot <appID> <depotID> <manifestID>
Download is silent and was low speed for me, but it worked, steam downloaded the old version to:
%steampath%\steamapps\content\app_appID\depot_depotID\
It will tell you when it's done in the console, including the path.
Now copy it all from your steam directory, then copy your GameData and saves folders into your new old version's dir.
Also don't be like me and trust old versions to remain as betas in steam :-)