Hi Alain,
Thats a tuff one , but , IMO--if the 9K is in good shape and your happy with it , I say keep it. I have a Y2K and like it but the 9K is a little better as you can up-grade it. On the other hand "Tyros" will be comming soon and the 9K may be no more. If you "gig" alot does the weight of the 9K bother you? If you decide to keep it make sure that it has the latest up-grade (you can down-load the up-grade from Yamaha.com)
The Y2K can`t be "software up-grade-able" a chip has to be in stalled. I have not had the up-grade done on my 2K as I don`t think it`s needed (imo)
Whats your "main - board" ?
I hope this helps , best of luck !!
jedi