If I am not mistaken, Rule 240 only applies to "Schedule Irregularities" weather, strike, war , acts of God, etc... In this case, United would be re-routing you do to a schedule change and not Rule 240.

I would seek a flight through Chicago, as ATL-ORD runs pretty much every hour, whereas there are only 2-3 flights a day to DEN from ATL. Currently there are ORD-LAX flights pretty much every hour as well. United should be fairly flexible in offering you a seat through Chicago. Don't hesitate to escalate it to the supervisory level, politely of course.