Iridium 5 *may* have been replaced by Iridium 51 on August 11, 2001.
However, the situation is currently confused. Elsets from the Orbital Information Group (OIG) released between August 11, 2001 and August 20, 2001 appear to show the same object being tracked under both identities.
See the discussion threads on SeeSat-L:
Iridium 5 moved to non-operational orbit
starting at http://www.satobs.org/seesat/Aug-2001/0244.html
and Iridium 5 & 51 "merger"
starting at http://www.satobs.org/seesat/Aug-2001/0296.html.
By August 21, 2001, Iridium 5 was being reported as back on station. See http://www.satobs.org/seesat/Aug-2001/0385.html.
So the constellation is complete once more, but this still leaves open the question of whether Iridium 5 remained on station all the time and was merely temporarily confused with Iridium 51, or whether Iridium 51 in fact replaced Iridium 5 on August 11. Anyway, at least, OIG is now showing Iridium 5 and Iridium 51 as separate objects once again! .