How to check if file has changed during (hook) form_validate in a form?
There are 2 approach we can look into:


Query the DB using db_query in field table, like if your field machine name is field_image, then its field will be field_data_field_image, and get fid of a particular node, in this case fid will be in field_image_fid

SELECT field_image_fid from field_data_field_image WHERE entity_id=NODE_ID


It will get u previous fid, and u can validate


Load Node from nid, either using node_load or entity_load_single, from here you will get your Old FID, as image has not yet been attached to the node, it has only been saved in file_managed Table.