Is there anyway to change the 'Display Data from' option in a field from one option to another based on an empty value in fk field.
I am afraid that's not possible, but you could place one field over the other and use conditional formatting to control the display (though not the entry).
You can display different data conditionally using a Calculation field:
isempty(manual_field) or manual_field = 0 ; fk_field ; manual_field)
Then you can place the calculation field directly on top of your manual entry field on your layout, then don't allow entry into the calulation field in browse mode so you can still click into the manual entry field to enter the data.