The Mir crash date and splashdown site have been altered to avoid uninhabited French South Pacific islands.
Mission controllers say the craft will probably come down on March 21 or 22.
New Zealand has started warning ships due to pass through the area set aside for the crash.
Vladimir Solovyov, head of Russia's control centre, would say only that the zone has been shifted at the request of "navy chiefs".
About 1,500 fragments weighing up to 40 tonnes in total may reach Earth - some at speeds fast enough to smash straight through two meters of concrete.
Air traffic controllers in Australia and New Zealand are in contact with space control to ensure safe routes for aircraft when Mir heads toward the Pacific,The Moscow Times reports.