If you can shapeshift, then after several thousand years, you are the only person that remembers what you originally looked like because you don’t use that form any more, and no one can force you into it via summoning, then my guess is that whatever your most common form is would sort of become your “defacto” new form.
But then again…maybe not?:-k
It would be interesting to see, anyone want to try to force Lillith back into her original form? If so, please let me know so I can be on the other side of the Abyss.