What if cavalry had the option to fight dismounted? Cavalry commanders would have the ability to dismount their troops, turning them into regular infantry with the same stats for the rest of the turn. If they are dismounted, and any battles would occur at the following turn change, the unit would fight as infantry. Units would only be able to mount/dismount once every turn (or every full day). I'm not sure if there is a difference in troop payment costs between infantry and cavalry with the same stats, but if there is, dismounted cavalry would cost the same as mounted.
I don't like this idea, unless there came some sort of penalty to dismounted cavalry. The thought is that they are trained to fight mounted, and throughout history, there were the fairly uncommon exceptions to cavalry that could fight well on foot. In recent history, there was that one guy in the Battle of Gettysburg who held out on that hill (Gee, my Civil War history sucks so bear with me).
Furthermore, the armor might be slightly different, as horseback riders will generally not want too heavy armor. Sure you can have this changed, but what if we introduce the idea that archers can also do the same? Then we might as well not have any set unit types in the first place and just call them "soldiers" at the RC, for the troop leader to customize.
So the point is, generally, bad idea to have convertibles unless this was balanced out some other way. Also it would mean we can't just define unit type variables by infantry, archers, mixed infantry, cavalry, special forces, peasants, daimons, monsters, undead, whatever else, but need a further special indicator for which infantry are obligate infantry and which are converted from cavalry.