How to override a deck?

I know how to create a custom deck, cards, etc. But recently i tried to override a deck. With override i mean replace an existing one provided by the core or by a DLC. In my case i tried to replace Ghoulkeeper with a custom version.
My first try was to copy F12_GETH.xml to a custom mod, make the changes i wanted, and use the new mod. This did not work, no matter if i kept the deck uid or if i change it, same for content pack, etc.
Does anyone know what is the correct incantation to do this?
I can probably open the data_dlc_004.wad, replace the file and be done with it, but i am hoping for a less obtrusive way, like when we fix a core card.
My first try was to copy F12_GETH.xml to a custom mod, make the changes i wanted, and use the new mod. This did not work, no matter if i kept the deck uid or if i change it, same for content pack, etc.
Does anyone know what is the correct incantation to do this?
I can probably open the data_dlc_004.wad, replace the file and be done with it, but i am hoping for a less obtrusive way, like when we fix a core card.