How to compare a subform lookup field that allows multiple entries when edited

How to compare a subform lookup field that allows multiple entries when edited

I have a form with a subform with multiple fields. One of the fields is a lookup field that allows a multi select.

 

On edit validation, I want a workflow to execute only when the entries in that subform field has changed. The old. function is not working as expected on the subform fields, can someone maybe assist in a workaround for this one?