SPACECOM exchanged the labelling of Iridium 90 and Iridium 91 on January 1, 2006.
This does not mean any change in the Iridium constellation.
The object being tracked under catalog number 27372 (currently
spare in orbital plane 3) is now known as
"Iridium 91" rather than
"Iridium 90", while the object with catalog number
27373 (currently migrating) is now
known as "Iridium 90" rather than
"Iridium 91".
This presumably addresses a long-standing mislabelling of these objects to bring the designations in line with those used by the operators of the Iridium system.