Qrew LegendI don't claim to be any kind of expert in new forms but assuming that there is not that native functionality in new forms then what you can do is have a rule that says when the record is saved change the value of a a new field called [Old Assigned to] to the value of Assigned to.
Then when the form is opened you will know the previous value. Then you can have a form rule to say that when the [Assigned to] is not equal to the [Old Assigned to] , then require reason for change. I think that you will need to make the reason for change field be one that logs changes, or else once the field change reason has been entered the first time it will not know to make them update the reason again for the next time it changes
Mark is correct - the new forms don't have a change tracking function, you'll need as Mark suggested to store the value each time the record is changed through either a form rule or automation or some sort and compare as he's suggested.
