Help with faction tweaking

[LordCurlyton]

Did you remove all references to the Casse in all pertinent files? On brief inspection: EBBS, descr_strat, descr_regions, campaign_descriptions, perhaps export_units and export_VnVs, EDCT, descr_character, descr_formations_ai, descr_names, descr_sm_factions, EDB, EDU. There may be a couple others.
You may want to check out Hax's Great Faction Swaps in the sub-mods section. He has released a conversion for the Casse (into the Belgae).
EDIT: This also doesn't include the various files you'd have to make, like a new campaign map/description for the main campaign screen, etc.