fillable forms

Converting fields from one type to another in PDFs is challenging. Basically there isn't a way to do it directly. You need to remove the old fields and then create the new ones in their place. You can do that manually, as suggested above, or automatically using a script.

I actually developed such a script that allows you to do it quite easily and it can be purchased from here: Custom-made Adobe Scripts: Acrobat -- Convert Form Fields Type

‌Remove the radio buttons and create the checkboxes.