You're half right as regards my year and model but you did head me in the right direction.
Foot off the brake, press the start button once to "Accessory" position, the speedometer lights up and the touch screen activates. In that ignition position the Flexible mode selection is locked out while the other maintenance settings are free to adjust. That's what I did in the previous description.
Press the start button a second time with foot off the brake to the "On" position and oil change can now be switched to back and forth between Fixed and Flexible. Flexible resets to 7,500 miles.
However, I can also switch between Fixed and Flexible with the engine running.
The manual strikes again. Nowhere is this unintuitive procedure described.
For me it is more a curiosity since I do not want service notifications. Good to know it works for when I trade it 10 or 20 years down the line.