I still have and love my old race MTB bike which runs a 9s drivetrain system. My Fc-M770 crankset is getting old and wondered if I could use a 10s FcM-780 without chaining any other element in the drivetrain. Could I use the middle and inner chainring from a Fc-M770 (I still have some spares)? Lastly, should i stick to the 9s chain or change to a 10s one? (I've read your informative article on chain compatibility). Thanks much!!