Well what about the good old "ent_morph(entity, string);" ?
If you have a pointer to your sprite just do something like this:
Code:
function changeSprite(_ent,_file)
{
ent_morph(_ent, _file);
}
//call like this:
changeSprite(mySprite_Pointer,"new_bitmap.pcx");
Not tested but should work.
EDIT:
Now that I've read Scorpion's response I guess that you could give "ent_morphskin(ENTITY* entity, STRING* filename);" a shot too.