I'm a bit rusty but here are some random thoughts. I'm sure someone else will be along in a bit to give you a much more coherent approach....
I think your first objective would be to do a comms audit, to find out who gets what, how they get it, what currently works, what people feel they are missing out on etc. You could do this by a survey, focus groups, inviting people simply to comment or a mix of all three.
Once you've got this, can then design a better comms approach giving colleagues the comms the way they want to hear them.
If you're looking at doing comms during a period of huge change, then I would say the key thing is to get "management" on board and committed to being as open and honest as possible.
One thing that i have done quite succesfully in the past with comms to several locations is to get yourself a comms rep in each location who acts as a two way link, both disseminating information onsite on your behalf and also giving you valuable on the ground feedback about how it is working. They also feed back local stories and information that you can use in your comms. You'll generally be able to find someone who is interested in helping you ike this, las it gives them good experience to put on their CV.
Sorry, this is all jumping around a bit but as I said earlier, I'm a bit rusty. Hope this helps anyway.