OIG/SPACECOM exchanged the labelling of Iridium 11 and Iridium 20 on January 29, 2004.
This does not mean any change in the Iridium constellation.
The object being tracked under catalog number 25577 remains part of the active constellation, but is now known as "Iridium 20" rather than "Iridium 11", while the object with catalog number 25578 remains spare in a lower orbit, but is now known as "Iridium 11" rather than "Iridium 20".
This addresses a long-standing mislabelling of these objects. It would appear that these objects got interchanged by Spacecom at some stage before one of the satellites (known to the operators of the Iridium system as "Iridium 20") was raised to operational altitude in 1999.