How do I edit fields after Extending Rights to PDF

I am creating a fillable form to use in our office and want my co-workers to be able to save copies of the form after filling it out. So under the "Advanced" pull down menu I selected "Extend Features in Adobe Reader". Now I can't edit any of the fields. When I try a pop-up box tells me to save a copy of the file but that doesn't seem to fix the problem of not being able to edit any of the fields. Anyone know how to do this?

Thanks. I'm using Adobe Acrobat 9 Pro on an iMac running OS X 10.5.5.