Disklavier

I've read some of the posts on upgrades of older Disklaviers.  Can 
someone give me some feedback on the costs associated with this?  
What does a Mark III (latest/greatest) have in features that can be 
added to older Mark II models.  How much would this cost?  I am 
looking at few Mark II pianos and wonder how much I'm going to have 
to add to make it the same (feature wise) as a new Mark III?  Are 
there some things that CAN NOT be upgraded?  Thank you.