Related Questions:
Why can't edit I edit the Field Type?
Can I edit my Customer Field Type?
Field Type is greyed out when editing
Answer:
When a Custom Field is already made, the Field Type cannot be edited.
If you must change the Field Type, you can create a new custom field with the desired type, transfer the data, then archive or delete the old field.
If you are deleting a Custom Field that is already linked to data in Event Temple, you must reassign this a new field.
Archiving will keep the record of the old custom field but not display it unless you decide to show all archived Custom Fields.