5. Can I use my current 9-speed hubs with the new 10-speed cassette (CS-7800)?
Yes, any 9-speed Shimano free hub (FH) can accept the new cassette. A spacer (provided with the cassette) must be used for adaptation.
i was just wondering what the spacer is exactly, if the new 10 spd specific hubs deeper then why a spacer? just wondering, thanx.