Larian Studios Forums Divinity: Original Sin 2 - The Modding Dungeon Divinity - Original Sin 2 - Modding General Need help (replacing Lohse's special ability)

Can't help with the modding approach, but there is a way to do this, if you're ok with going about it the cheating way.

Cheaters have figured out a way to enter GM mode in the campaign, and through GM mode you can add and remove skills from your party members. There are two ways to achieve this, either by paying for a Cheat Happens account or using the free Cheat Engine software.

Cheat Engine:
http://fearlessrevolution.com/viewtopic.php?t=202

Cheat Happens:
https://www.cheathappens.com/21767-PC-Divinity_Original_Sin_2_cheats

Third way is by using a cheat mod called "Origin and Racial Spell Skillbooks", which can give you all the skill books for origin and racial skills. This however can't remove the original skill:
https://www.nexusmods.com/divinityoriginalsin2/mods/70/?