Question about putting a form into a lightbox like JS popup

I’m doing a little maintenance on a website where there are already two form pages that submit and do stuff via PHP.

My task is to take them and put them into lightbox like popups so they can be called from various pages.

I’m familiar with Ajax but I’m wondering if on this occasion it would be easier/possible to simply load the form pages themselves into the popups and have them stay and process there. That would mean I only need to load the existing forms into the lightbox, without having to rewrite the form logic using AJAX.

The only way I can see this working is if the form pages are loaded into iframes, inside the lightbox.

Has anyone done this? is it acceptable to do? or am I better to rewrite the forms totally.