Can I change a member's event form data after they have purchased an event ticket?

Yes, you can.  You need to go to the member's page in the admin panel, and you can edit the information there.  You can also view the form data via their order page, but you can't edit it there.

Member Page

From admin home, type the member's name or email address in the quick search box, to go to the member's page in the administration panel. 

Scroll down to the Tickets section.  Click on Edit next to the relevant ticket.  Don't forget to save once you have made the changes.  Note:  This will not automatically resend the ticket data to the member.   If you need to send them a copy of the updated ticket, you can click on the pdf link, and send them a pdf of the ticket information.

Tickets On Member Page - click to enlarge Edit Ticket Form Data - click to enlarge 

Order Page:  View Only

You can also view the ticket form data on the order page, but you can't edit it there.  The reason for that is, the order is a snapshot of what the member actually put in the form.  If you change the form data on the ticket (via the member's page), the order will still retain a copy of what they actually completed.  If there is ever a question of whether the member actually ticked that consent box, you will always be able to go back and check the order, knowing that it has not been changed by someone.

Order Event Form Data - click to enlarge