[jessriedel]

You've given basically the right answer for changing phase within the same orbit, i.e., if you start with multiple satellites near each other on the same circular path, and you want to space them out evenly along that path. However, the Iridium Next satellites have enough fuel to change ('migrate') to the next orbital plane over. They save fuel by using some tricks involving the non-uniform nature of the Earth's gravitational field, which only works because the migration can be done slowly.

There are 6 planes in the constellation and the satellites were delivered 10-at-a-time on 7 launches, plus 5 satellites on an 8th rideshare launch. There are 11 operational satellites in each plane (plus 9 spares distributed over the planes), and it's easy to see that migration is necessary to fill out all 11 slots in all 6 planes given those launches.

More info:

http://www.rod.sladen.org.uk/iridium.htm