simplistically - the bodyguard unit is chosen either as the first unit listed in that general's army in desc_strat.txt or if generated in the game by the first unit that is available to that unit (reading down from top of export_descr_units.txt) that has the general_unit attribute, so if you move the general_unit attribute to an infantry rather than a cavalry unit for that faction that is what you should get....

(there is also an upgrade that happens in RTW where I think the second unit with the attribute is used after upgrade - not 100% sure on that one, its not something I've used)

And DO NOT post question threads in the Scriptorium, Thank you,
you are about to be moved to Modding Questions where they should be posted...