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The how you do this actually belongs in the modding questions area, though why you'd want to mix and match the units might belong in here!

You don't need to change the texture at all, you just need to make sure you also copy the model from XGM into the EB game as well. (Thats the .cas file)
In case the EB model is used for more than one unit you might need to separate the descr_model_battle.txt entry for the spartan hoplites from any other units that are using the same model. Check if the descr_model_battle.txt entry has texture lines for any factions that you don't want to use the XGM textures.

type greek_hoplite_spartan_xgm
skeleton fs_spearman, fs_s1_swordsman
indiv_range 40
texture greek_cities, data/models_unit/textures/UNIT_GREEK_SPARTAN_PHALANGITE.TGA
texture romans_brutii, data/models_unit/textures/UNIT_GREEK_SPARTAN_PHALANGITE.TGA
model_flexi_m xgm/data/models_unit/unit_greek_spartan_phalangite_high.cas, 15
model_flexi_m xgm/data/models_unit/unit_greek_spartan_phalangite_medium.cas, 30
model_flexi_m xgm/data/models_unit/unit_greek_spartan_phalangite_low.cas, 40
model_flexi xgm/data/models_unit/unit_greek_spartan_phalangite_lowest.cas, max
model_sprite greek_cities, 60.0, xgm/data/sprites/greek_cities_greek_spartan_hoplite_sprite.spr
model_sprite romans_brutii, 60.0, xgm/data/sprites/greek_cities_greek_spartan_hoplite_sprite.spr
model_tri 400, 0.5f, 0.5f, 0.5f
The path to the models is bit I marked in bold - you either need to change EB's descr_model_battle entry for your unit or copy and re-name the XGM model to same name as EB's had and put it in the relevant folder.
You'll need to do the same sort of thing for the sprites (marked in italics) or comment them out by putting ; at the beginning of the line, otherwise your unit will change back into old one at a distance.

If you want to change textures for any other purpose you need the nvidia dds plug ins: http://developer.nvidia.com/object/n...ure_tools.html