I'm sure I've had sleep apnea at least 5 to 6 years before diagnosed. I could only sleep on my side, if I tried to sleep on my back, I felt like I couldn't breath, so would roll back to one side or the other.
Now, on CPAP for 8 months, and wow, I can sleep on my back for short periods of time. I still prefer side sleeping, but have hip and shoulder pain, so then I roll over and park myself on my back without realizing it and fall asleep.
My AHI goes up a tiny bit while on my back, but having an Auto machine helps. That's what it does best, adjust the pressure as needed during the night. You pay a bit more, but isn't your sleep quality worth it?
If I stay on my side all night, AHI is always around .7 or under.
If I find that I have been on my back, my AHI is usually at 1.2 to 1.7.
I don't mind the rise in AHI, because it is still very low, and my comfort comes first!
So, don't dismiss the auto machine. If your want to use it at a straight pressure, you can. But you can't change a straight CPAP machine back to Auto.