Disklavier

I am the same person who is considering getting the DC3 with the 
Mark IIXG system in it (the paino says NP98 on the bottom, so I 
assume it is year - 1998 - the piano was born).  

Anyway, if it has a MarkII-XG platform on it (it also has the CDrom 
strapped onto it) - can you simply buy the MarkIII box and plug it 
into the piano or is there something more to it than that?  Is the 
newer box only compatiable with newer painos or is the hardware 
control in the painos the same?????

If it is the same how does one go about getting a mark III box and 
how much do they cost?

THanks again for all your responces.  This is truly an unbelievable 
group!