We can change the guest or sponsor to a registrant but cannot change a registrant to a guest. 

Alternative solution: 
  1. Delete the Registrant
  2. Recreate the Participant as a Guest.
For more information on changing the participant type, please review: 
Please visit this topic on our User Voice to vote and/or add comments as necessary.  Topics that receive votes in the User Voice have the opportunity to be included in our Discovery program, which determines what functionality is added into future builds of The Raiser's Edge 7.