When another registrant is registered and on the roster for the activity other than the intended registrant the participant can be changed without having to mess with any financial data.
To change participants go under the user profile, select voids, refunds, transfers then next to that transaction select the hamburger menu (three horizontal lines-stacked) and select "change participant".
On the next screen select the intended participant under "target participant". Review transaction data, preview the transfer and transfer the participant.
Note: You can only transfer participants that are on the same user account. You cannot switch from one family account to another. If this is the issue please contact CivicRec Support for assistance.