To promote an existing TeamRaiser registrant to Co-Captain of their team you can follow these steps:
  1. Go to Fundraising > TeamRaiser
  2. Click Manage next to the appropriate event
  3. Search for the participant name
  4. Click on the participant's name from the search results list
  5. Click View Registration record
  6. Click Change Team Membership
  7. On the change team membership screen, select "Promote to Co-Captain of this team"