I too am an ADBY - and I have discovered that my ancestors used ABDY as well. Whether by design, error or fancy, I'm not certain.
I am interested to learn that you have evidence relating to this as I don't seem to be able to find them before 1800.
So far I have: James ADBY and Rachel HIZZEY, Richard ADBY and Elizabeth HEAVER, George ADBY and Alice ADNAMS, Charles ADBY and Esther BUTLER ( my grandparents).
I would be very interested in the proof you have and am willing to exchange any information I have to add to yours.