You may need to refund part of the registration fee for one of the following reasons:

  •  A participant dropped out of a multi-participant registration and wants a refund for their ticket
  •  The registrant chose the wrong ticket type during registration and paid too much
  •  The registrant failed to use a discount code that they were entitled to

You want to avoid asking a participant to re-register and the existing participant may already have sponsorships

What to do:

Cancel and refund the full registration fee. Our system does not allow partial refunds.
Then, leave the participants that are still participating as they are, and cancel any participants who are no longer participating. 
Note: Refunding registration fees does not affect participants. 
Finally, have the registrant make a donation for the correct registration amount to their participant page.

The solution for processing registrant refunds and possibly cancelling participants is here:
Wizathon: Processing Registration Refunds and Chargebacks. Participant Cancellations