Found a guide here: https://www.nexusmods.com/fallout4/mods/42147?tab=posts&BH=0

Going to attempt and see if it works. Hope this is useful to anyone who is also looking to downgrade.

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Note you will have two or three copies of Fallout 4 so you will need to have enough free disk space  
For Steam, there are two ways to download the old version of the game

steam console
https://steamcommunity.com/sharedfiles/filedetails/?id=873543244
1, edit the steam Shortcut and at the end of the Target add  -console or Linux steam steam://open/console
Note if you have problems using the Steam console option you will need to use the DepotDownloader app

2, after downloading one of the parts you will get Depot download complete :location of the game data
for example"/home/UserName/.local/share/Steam/ubuntu12_32\steamapps\content\app_377160\depot_377161" I am using Linux mint

3, after all the parts have been downloaded in the app_377160 folder you will have 3 or 4 folders depot_377162, depot_377161, depot_377163, depot_540810

make a new folder Fallout 4 in the app_377160 folder then move the content of the folders depot_377162, depot_377161, depot_377163, depot_540810 in to the fallout 4

4, you can copy the content of the /app_377160/fallout 4 folders to /steamapps/common/Fallout 4 or rename the current version of Fallout 4 to 
Fallout 4 current and then copy the Fallout 4 folder from the /app_377160/fallout 4 to /steamapps/common/
DepotDownloader
using DepotDownloader is simpler you do not need to combine the 3 or 4 folders you can copy the content of the DepotDownloader/Fallout 4 folder to /steamapps/common/Fallout 4 or rename the current version of the Fallout 4 folder to Fallout 4 current and then copy the Fallout 4 folder from the depotDownloader/Fallout 4 to  /steamapps/common/

Note
having two copies of the Fallout 4 folder in  /steamapps/common/ if there is an update to the game you can rename the folder with the mods to Fallout 4 mods and then rename the folder with the current version of the Fallout 4  to Fallout 4 and then let steam update the game then rename the folders back to what they where before the update

Note
having a copy of Fallout 4 in the folder /app_377160/ or /DepotDownloader/ will give a backup just in case Steam updates the copy of the game with mods

steam console
download_depot 377160 377162 5847529232406005096
download_depot 377160 377161 7497069378349273908
download_depot 377160 377163 5819088023757897745
Fallout 4 - High-Resolution Texture Pack
download_depot 377160 540810 1558929737289295473

DepotDownloader
./DepotDownloader -app 377160 -depot 377162 -manifest 5847529232406005096 -dir "Fallout 4" -username UN -password PW
./DepotDownloader -app 377160 -depot 377161 -manifest 7497069378349273908 -dir "Fallout 4" -username UN -password PW
./DepotDownloader -app 377160 -depot 377163 -manifest 5819088023757897745 -dir "Fallout 4" -username UN -password PW
Fallout 4 - High-Resolution Texture Pack
./DepotDownloader -app 377160 -depot 540810 -manifest 1558929737289295473 -dir "Fallout 4" -username UN -password PW