Checkboxes field to Radiobutton

Hi,

I've a problem with a field in a form.

This field was a radiobutton field, One day I edited it (to make a proof) and I change it to a Checkboxes field.

The ploblem became when I tried to edit it another time to change it back to radiobutton type. I don't know whay but i Can't change it back to radiobutton type.

It's important to me to change it to radiobutton because there's a lot of scripting behind that need it.

Anyone can save my life???

Thanks a lot.