Form Inheritance

I need three forms on a site, a Registration form, a Volunteer form and a Contact form. I developed the Registration form first, as it was the more complicated. I then saved this form, did a save as for the Volunteer and a save as for the Contact form. I then Edited the Contents of the Contact form, removing the surplus fields and validations, etc. and saved it. Now when I go back into the Registration form, when I Edit Contents, I find I only have the fields of the Contact form, even though they are visible when I do a Live View. If I save the Registration form after editing the contents, I then loose all the fields of the Registration form except the fields in common with the Contact form.

Nearly all of my work with the Registration form has disappeared. How can I avoid this?