For well over 15, maybe 20 years, I was a side/belly sleeper. Just couldn't go to sleep on my back.
25 years ago, I was mostly a back sleeper; that changed over time to being the dedicated side/belly sleeper in spite of the more comfortable (at least more comfortable for my back) supine position. I suspect the change had to do with weight gain and gradual onset of apnea through the years (Some girls told me that I SNORED - LOUDLY AND A LOT - when back sleeping). Side sleeping probably some subconsious defense mechanism as a way to moderate the effects of apnea which are often worsened or made more pronounced by supine sleeping position.
Well, for the last ten or fifteen years, I have been totally UNABLE to fall asleep on my back. Sleepy, go to bed, put on CPAP, lie on back miserably wanting to go to sleep for an hour, sleepy as all get out, but unable to sleep. Roll over, fall asleep in less than five minutes. (Honestly, my interest initially in CPAP therapy was partially an attempt to reclaim the supine sleeping position).
Well, a couple of weeks ago, got to having some bad back pain and the only position I could find that was not acutely painful was flat on my back with a pillow bolster under my knees, so decided to just lie on my back even if I got no sleep. Took a while to GO to sleep that first night, but back was very painful on side, worse on belly, so those were out. Finally fell asleep. Next night on back was better. And thus far, I have slept on back all night, every night. Less moving around and better dream activity (at least I am remembering dreaming even if details escape me).
Expected more AHI events and higher pressures for longer time, but am just not seeing it in first couple of weeks. Will check again this weekend and see if there is any trend away from that.