There is no major difference in the Core Content and Theory of NCERT Biology 2019 and 2020 Editions, and you may definitely refer to former edition for educational purposes. A new NCERT edition has hardly few minor changes in exercises and revision of errors in them. According to a Circular by NCERT in April 2020, they had made 1334 changes in NCERT Books, at an average of 7 changes per book, which may be considered. You may take note of the changes later if required.