SPACECOM exchanged the labelling of Iridium 90 and Iridium 91 on January 1, 2006.
This did not mean any change in the Iridium constellation at
that time.
The object being tracked under catalog number 27372 (at that time
spare in orbital plane 3, but since brought into operational use
to replace to destroyed Iridium
33) is now known as "Iridium 91" rather than
"Iridium 90", while the object with catalog number
27373 (launched to orbital plane 3, but
later migrated to orbital plane 5) is now known as
"Iridium 90" rather than
"Iridium 91".
This evidently addressed a long-standing mislabelling of these objects to bring the designations in line with those used by the operators of the Iridium system.