One question to ask is : Does Access Know that you changed the field from String to Autonumber?
One way to be sure is to drop the textbox from the form and put it back on (close access before doing this).
You can always code something in the after_update even of the form to refresh the id column so that people don't have to go to another record to get that id.
You'll also be able to refresh the combobox of the master form above so that the new id is displayed there as well.