I think the date of Orthodox Easter changes like the Western Easter does, only it operates on a different calendar, Julian, I think.
In 325 AD, the Council of Nicaea decided that Easter would be observed on the first Sunday after the first full moon on or after the March equinox. While the Council of Nicaea set the Easter date for most churches around the world, not all Christian churches observe Easter according to the Gregorian calendar. Many Orthodox churches still witness Easter in congruence with the Julian calendar.