Once you have added an attendee custom field to a ticket and registrations have begun, you won't want to remove or make changes to that ticket.
Doing so will hide some of the data that was submitted with that registration.
How to change a registration form for a ticket already in use:
If you do need to make changes to a ticket that has already been used, we recommend the following:
Make a copy of the ticket you want to change
Update the new ticket with the proper form fields
Set the display settings on the original ticket to "admin only"
This workflow will result in two different ticket types in your records. However, you will retain all of the data associated with those registrations.