Question: are 9 and 10 speed cassettes interchangeable?
Answer: Shimano 8- 9- and 10-speed cassettes/hubs are fully interchangeable for wheels with steel Freehub bodies. The only parts that are different in any important way between the 8- 9- and 10-speed systems are the shift control levers. ... (Standard Ultegra hubs have steel Freehub bodies.)