Each creature has not only geometry but also a file that, let's say, describes the path to its location and sets some settings. This file is what you need; it will indicate the path to the geometry you need to replace. It usually has the following format: "creature name"-geom.xdb. In your case, it is located in Characters\Creatures\Renegades\ and is called Champion-geom.xdb and Champion-geom-AI.xdb.
Champion-geom.xdb describes the creature's geometry in the castle/recruitment window/battlefield.
Champion-geom-AI.xdb describes the creature's geometry on the world map.
Open the Champion-geom.xdb file, and you will see:
25E7AA41-C3A0-41A7-8D53-409E08705CF0
Global
....................................................
We are interested in the geometry ID, which is in the line
25E7AA41-C3A0-41A7-8D53-409E08705CF0
This ID is responsible for the creature's geometry. To change it, simply find the geometry file with this ID. All geometry files are stored in the data.pak\bin archive. Just find it and replace it with the geometry file from Lords of the North, but remember that the file name must match the ID specified in Champion-geom.xdb. Don't forget to do the same with Champion-geom-AI.xdb; otherwise, the creature will not be displayed with changes on the world map.
But... what do the Knight creatures (Isabelle's creatures) have to do with it?
I need to change the Heroes - Knights, like... Andreas, Lorenzo, etc