Hello @anon78312334, that is currently not possible. Here is an existing feedback request for you to upvote: Change field type once in use
Was data entered in this custom field before? Or the custom field did NOT have any data entered across the project?
Did you delete the custom field completely from the org or you only removed it from the project?
If you only remove it from the project but data was added before removing it then it normally shows data like this:
And in subtasks custom fields show like this usually: inherited fields
