As of mid-October, 2005, Iridium 91 (27373, 2002-005B) [subsequently relabelled as Iridium 90] started manouvering.
In mid-October 2005, it was raised from the parking orbit (673 x 670km) to the engineering orbit (754 x 743 km), and its orbital inclination was increased, so that it drifted away from orbital plane 3.
The intention was clearly to drift it to another orbital plane. Orbital plane 5 had been without a spare satellite since Iridium 86 (25528, 1998-066B) took the place of Iridium 16 (24841, 1997-030F) in the operational constellation in April 2005.
27373 arrived at orbital plane 5 in January 2008.