Detect a change in primary contact using business rules

Summary

Need to trigger email to the new contact when the primary contact on an incident changes

Content

We have a requirement where, if the primary contact on the incident changes, then an email should be sent out to the new contact informing about the incident assignment.

I tried to use business rules for this, but the if condition in the rules does not give access to the contact id field on the incident. 

How to detect a change in the primary contact of the incident in business rules?