Do the Airpods Pro change how your internal voice sounds?
No they do not - not due to plugging your ears and not due to loopback.
Both AirPods and AirPods Pro don’t loop back audio, so you don’t get any negative effects whatsoever. You have to go out of your way to loop the audio back with a tool like LoopBack to get any hint of your own vocalizations in the mix.

https://www.rogueamoeba.com/loopback/

AirPods Pro can be even better in transparency mode if you don’t like to have any attenuation. The Pro on full isolation mode are less effective than Etymotic ER-6 and more effective than foam noise protectors in my experience in terms of external noise and less impactful to making you think your voice is different.