Actually a planted bomb is a "grenade". And its EV_SZ_model is "models/w_c4.mdl".
entity_set_model() probably works to change its model.
Or you could try to hook FM_SetModel and when it says "models/m_c4.mdl", you just replace that with your new model. Probably a nice way to do it if you intend to change the model at all times.