Currently running a saint 10speed mech with clutch, on a 9 speed cassette with a sram x-0 9 speed shifter. So yes, super easy to do without having to replace everything. I didn't even need a spacer on the cable clamp like some people have, just attached it with a new cable and good to go. Been fine for quite a while now.
Also, go for a clutch. Best little things that have happened since hydro brakes