Another way to do it is you keep most of the renders as the girls look now and make the new form a combat form, kind of like what happens to the MC, but they are in control of themselves. This way the rendering load wouldn't change much. Then you can just use the combat form in places like the dojo during practice or in scripted story events. I think in this way you get to do both forms without overloading yourself with rendering everything 3 times and we still get to see the new form.
Also doing it this way the current intimate scenes will still make sense having the original form for the girls. You can always add new intimate scenes for the girls in the new forms when you have time.