This is just my personal experience so please take it as such though it may help some others in the same odd situation I was and am.
Diagnosed with moderate to severe OA. O2 levels drop into the low 60s very fast with only a few close events. At the lab faster than they had ever seen before so they said. But on the machine and titrated O2 stayed in normal range. Titrated pressure was 15 using a fitlife total face mask.
Doc sets me up with an auto and I get a PRS1 series 60 auto aflex. Docs settings 6 min to 15. I request a bit higher min because Im smothering at 6 in the Fitlife so Doc ups BOTH min to 8 and max to 18 This is all months ago at the start of the trip down cpap lane.
Fast forward a month. I am gifted a old nasal mask and after that a Simplus FF. I nasal a few nights and find AHI goes WAY down and so does the pressure the auto is running. Like a reduction in max pressure from 15 to 16 at times to never going over 11.
Try the Simplus FFM. Love the mask. AHI goes up a bit and pressures go back up, with the machine saying start at 10.5 min. Might have done that with the Fitlife also but I didnt know how to turn on the Opti start feature when I used it.
Still Fitlife goes in a bag and I go with the Simplus for several reasons. Nasal is too small so wife starts trying it but doesnt get along with it so it goes in the bag too LOL.
Then Doc says ok you know you can use a nasal so thats what we do. And I get the Eson which is perfect. Seals loose, no leaks, if its tight enough to stay on your head itll seal so im sold on it.
And now the interesting part of my little cpap trip. With every mask my pressure requirements are different. Max needed is way less with a nasal than with a FFM And FFM is less than with a Total face.
Point im driving at is if you were as I was titrated with say a Fitlife or the like or even a FFM and then you switch to nasal and you are on straight cpap its entirely likely you titration pressure for the FFM or TFM is much higher than it needs to be with a nasal mask or pillows. Or vice versa if your titration was done with nasal and now you use a FFM it could be to low. Auto it wont matter much except that your min pressure could likely be set lower also or need to be set higher.
I realize this is one persons experience and everyone is different. But with me and the wife the change in pressure and AHI vs nasal FFM and Total Face is as predicable as clock work. And consistent as the sunrise.
Just something I thought Id toss out there in case it happens to be of use to anyone.