When I need to do something similar and I want to trigger a Form Rule off of a value changing in a field I often create a log field, so in your instance something like WR# Historic that I can set up an Automation to say whenever a record is added or edited and that field changes update that WR# Historic with that new value. This way when you change it on the form you can compare it against the field that has the historic value to trigger the Form Rule. Then once it is saved it will update to the new value making it ready to be used again. It does mean adding another field and Automation but it can help you get that value in a place you can use it to compare in Form Rules and trigger off the requirement you need.
