Yah, that’s always technically a factor, but if you have to doubt your own basic perceptions your ability to hold many (if not most) beliefs is severely damaged. It becomes difficult to invest yourself into the world you appear to exist in because it “might not be real.” And frankly that’s really boring, and I can’t live a good life leaning on that doubt.

The best one can do is trust their perceptions (unless something is obviously affecting it like substances) and assume their reality is real. If they later “wake up” in the “real” reality and learn the reality they knew was false, then mourn what wasn’t but seemed to be and start living in the “new” reality.

In this case, if the new identity perceives themself as a direct continuation I don’t see a compelling reason to doubt their perception.