Interesting stuff. I assume they have a way of bringing the old Iridium satellites down to burn up ?
Yes, mostly. The plan was to de-orbit the old satellites by moving them into a low elliptical orbit (250 km at the perigee). The atmospheric drag at this altitude will eventually slow them enough that most will re-enter the atmosphere and burn up within several months.
The problem is that many of these old satellites are very nearly out of fuel -- because of delays and the need to quickly switch them to different orbital planes than originally intended.
So instead Iridium is proposing to place these satellites into an elliptical orbit with 600 km perigee. From this altitude it might take up to 10 years before the satellites will re-enter the atmosphere.