I noticed on the Prius Model IV with out Nav I have a AM/SAT button that switches between AM and three SAT modes SAT1, SAT2, SAT3. I looked up the features in the manual and it claims the button switches between the three modes and nothing thing more. I am guessing it can work one of two ways. 1.) You can have XM on SAT1, Sirius on SAT2, and a HD radio add-in on SAT3. (I hope it works this way). 2.) Or presets for SAT channels. It is an odd way of setting three satelite presets. (This would be foolish) It should use the 1-6 preset buttons for that.
I just added Best of Sirius to the XM plan and called it done. I have no idea what you'd have to do to convert completely from XM to Sirius and keep the integrated look and feel.
This is what we plan on doing, just getting XM Lifetime w/Best of Sirius. We figured it was cheaper than buying a Sirius brain or whatever and having it installed. We'll just have to re-learn where all the channels are.