Wow, that is awesome.
I wish I could have found that a little earlier. I was looking around in the database using phpmyadmin. I deleted an entry in the cck_core table that looked like a duplicate entry. tried setting the admin template for the item in the cck_core_types table template_admin field, and that was able to bring up a form, but I could not get any changes to the form to stick after that.
I just deleted all my content types and am recreating them now. That seems to have worked as I can now bring up the recreated form. The form version feature is a nice thing to have.
I'm curious to know if it was the category change that caused the problem or the change in a field type.