Unfortunately, the answer is no, you cannot change the format of these fields. All you can change is the name. If the format is date, if you try to put any data in that field other than a date – you will receive an error message that this cannot be done. Also, if you do rename the field another downside would be that the standard title of the field still appears in filters even if you edited the name to something else.
An alternate method would be to use the custom fields in either the item or customer list as these fields can be free text. You can then add these fields to the invoice template.
As always, let us know if you need additional assistance on this topic or any other QuickBooks questions.