Ummm... in my case I don't care.
If my 5★ waifu can be promote so she get promote.
Even she full on skill lv and slot she still get promote.
Because she will be more helpfull in front line.

I may use my bone to find her missing skill level.
I may use my blood to find her missing equip slot.
Nor death nor life I will bring her back to her true power, and I will withstand any pain doing it.