Very carefully. Many of the ancient units have special art according to what category of civilization it is from, so you have SouthAmerican swordsman and Middle Eastern Horse Archer and so forth. These are in a file in the Civillizations folder called
CIV4UnitArtStyleTypeInfos
If this file is messed up you don't get a regular xml warning or "this line is messed up I can't load that file, oops the other files needed that one". No it just gets you surprise CTDs. So, you have to take these units out of UnitArtStyleTypeInfos, UnitArtInfos, and the UnitInfos and UnitClassInfos. Also many of them are referred to by other files, such as Warrior being a goody result, and Minor Civs having the default Archer as their unique archer and such whatever that stuff is about. These other will get your regular xml load warnings, so you should be able to find them. To obviate all this you could use the name UNIT_WARRIOR and have everything else about it be a Tank or something. This will fool the system so all the little nagging files don't have to be fixed, but is hardly worth the trouble, and might have all kinds of unexpected results such as tanks out of goodie huts (if they exist in Korea).
In fact, come to think of it, what your are talking about is a mod, not a scenario.