How to Cancel/Reset the existing registration.
Once someone has started to register for an event, their registration is already set up in the system. So even if they join the chapter, they will still just see the original price.
So you should go back into the event roster, find the registration and cancel it.
First find the event on the website, and go to the event page
When you are logged in you will see a blue "Edit" button. Click on the down arrow to access the registrant roster report
When you find the registrant - check that they have not already paid.
Use the button on the right to find the cancel option, and cancel the registration.