Hey richards1052,
Thanks for stopping by the forum. I'll be happy to help out with converting field types.
Within MailChimp, the checkboxes and radio buttons are actually a special kind of field called Groups in the MailChimp list. While it wouldn't be possible to change a field or Group type once it's been made, it would be possible to create a second grouping in the list of the desired type. Once that copy of the groups has been made, you can move the subscribers over from the original groups to the corresponding new ones. For more information on editing groups in a list, check out this guide: http://eepurl.com/gWOb
Once the Groups have been updated in the list, you'll want to refresh the list in the WP plugin settings. To do so, head into the WP Admin page, and then click on Plugins> MailChimp List Subscribe Form > Settings. There, clicking the "update list" button at the top of the page will refresh the list with the new Groups and fields.
Thanks again for posting. If you have any other questions, please feel free to reach out to us again. Have a great day!
-Gunter