Highly unlikely it's an equipment swap. The R availability flucuates on a daily basis. It's not unusual to see R9 today and then R0 tomorrow and the cabin still appear to be empty. Lesson learned by many of us is that when you see R>0 don't wait at all.
As a sidenote, are you swimming in RPU/GPU's? The PHX-DEN flight seems like a waste of an upgrade, especially a GPU. Just my unsolicited opinion.