Usually orbits are reserved years in advance. AFAIK this kind of move is unprecedented (although everything about Starlink is unprecedented since it's the first megaconstellation).

Two weeks ago, a Starlink satellite exploded. SpaceX believes it wasn't caused by a collision which means the explosion was probably caused by a malfunction in the satellite itself. Now 4,400 Starlink satellites are moving to a lower orbit for "safety". Is this an emergency change to account for a design flaw that they just discovered?