At the moment, when you create a field - you cannot update that Data Source after it is created. I can understand if it was a Shared Field and used in multiple forms but if it is a field that is only used in a particular form it makes administration just that bit more complicated.
What if the wrong Data Source was selected or a Custom Field was created that was better suited for that field or Freshworks make a product update that was more suited for that field. It cannot be updated and therefore would have to be deleted and replaced with a field that had a slightly different name (then maybe updated to the same name after the previous field was deleted and form saved). Having the ability to update the fields Data Source would make more sense and easier for administrative purposes.