Briefly,
First, any current user from 851 to 853.9875 will vacate the current frequencies. These will be relocated to 854 - 859.9875.
Any current public safety user that is 860-860.9875 will have the option to relocate to the 854-859.9875 portion of the band.
All licensees on 866 - 868.9875 will be relocated down 15MHz.
Any conventional, LTR, or EDACS system will only require reprogramming (and perhaps antenna related work).
Any Motorola system (other than a very new system) that moves to 851-853.9875 will require reflashing or replacement of the radios and other hardware.
Nextel vacates all licenses from 851 to 861.9875 and gets the just cleared 866-868.9875 spectrum.
There is also considerable rebanding with TV equipment (remote pickup I believe, perhaps in the 1.9GHz band) This will not affect the scanner world.
I think that is a basic summary.
chris