QFC - how do you validate and then buildForm() ?

Hello, This is my first post to this list (I think). I have a custom QFC action whose effect is to alter the way the form is built. I do not want to process the action unless the form is valid though, but this requires me to build the Page first using Page::buildForm(). This poses a dilemma, as no matter what I try I can not seem to rebuild the Page differently to the way it was first built. Does anyone have a workaround? Much appreciated. -m