Stars do change their brightness in various other ways, but the light curve of planetary transit has a very characteristic shape. It causes the brightness to dim by a small but constant amount, with a (comparatively) very short and sharp start and end. A transit causes this pattern to occur at precisely regular intervals, and I don't think we know of any phenomena related to a star itself that would imitate the same effect.

Stars' relative positions generally don't change fast enough for the angle from which we observe a transit to change significantly. A transit of HD 20794 d is visible anywhere within a roughly 0.7-degree wide band. But our angular rate of motion with respect to the star HD 20794 is the same as its rate of motion in our sky, about 0.001 degrees per year. So the transit will most likely continue to be observable for decades or centuries to come, depending on exactly how the planet's orbit is aligned.