Update custom field based on another custom field in Test cases

Hi, I have custom field in Test cases as "Automation" (Checkbox). If the test cases is automated, it will be checked else it will be blank.

Now we want to modify the field as Drop down with value Yes/No/NA. So I added new custom field in Test case as "Automation Status"(DropDown).

But the value is blank for the dropdown in all the test. I want to update the value of dropdown based on the checkbox value. (Eg: Yes should be set as Dropdown value if checkbox is clicked).
We have 500+ test case which is difficult to update manually. Is there any way to update the dropdown via automation rule or something?