It’s something that the die-hard Marvel purists may take issue with, but there is a very smart explanation for the change: in order for The Ancient One to properly reach and teach Doctor Strange, Tilda Swinton decided that the best approach would be for her to have a personality that mirrors the hero’s.
The version of The Ancient One played by Tilda Swinton in writer/director Scott Derrickson’s Doctor Strange isn’t exactly what you would call a perfectly accurate representation of the character of the same name from the comics.
In the source material, the Ancient One is an old Asian man who does a whole lot of sitting and meditating — and Swinton’s incarnation is not only a woman, but also far more spritely and active.