Not sure I understand what you are asking about the PO2 sensor. If you don't have the sensor on your watch, you can't flash it to have that.
I did the flashing a couple of years ago and it's worked great. Not a problem. I was wondering the same if there would be anything that they might have changed. My understanding (not an expert at this), you just flash whatever firmware you want. You can flash an old one and the watch will update to the latest one. In other words, Garmin doesn't know if you did the flashing or not.