Hi Telmo,
If you have memory cards , no problem ! > link:
http://www.sandisk.com/Products/Item(1151)-SDDR-99-A15-SanDisk_ImageMate_5In1_ReaderWriter.aspx

Yes , all your saved styles would work with the PSR-S900

Yes , the key-bed is the same , NO change.
It might feel like a change at first , but that is because it`s new , after it is used awhile and loosens up it will feel like the Y3K

I think the S900 has more user memory , if not it will be the same, ( I think more )

Now i must tell you the bad , at least for me.
The small rubber buttons on some units tend to stick , not so has to cause any problem , just a pain that Yamaha didn`t use plastic buttons like your Y3K.

So in the end:

Better vocies
Needs more mixing & tweaking to balance the sound.
Updated style is a plus
Key-bed NO change
Rubber buttons
NO -- USB up front , but like BK said" use an extension cable"

Link for cable:
http://www.monoprice.com/products/produc...&seq=1&format=2

I hope this helps !!
Gary