When i bought my car, my SA, who I've used for years and has always been up front with me told me the only changes the M5 is going to have are:
New nav software(already on all 2013's)
The ability to download updated software OTA (already on 2013's)
New idrive controller that you can use your finger to write with, similar to Audis system.
Front bumper will recieve a small modification, seemed like just a small bar goign across the middle opening on the 5sport bumper, not sure if M5 is getting it
Angel eyes will now be more sqaure than round.
Turn signals will move to the "eyelid" as well as on the side view mirrors.
Considering all this I decided I would get the car now. Spoke with a tech and he told me from what he has seen, 2013's that have the new nav software should be able to upgrade to the touch idrive controller by replacing the controller. I tried Audis before, and i didnt care for it much.
Also, if you have the option where your car warns you when someone is in your blind spot, the turn signal side mirrors are a simple swap as well.