Having replaced most of my drive train over the past 3 months my rear shifter has given up the ghost.

I currently have 9 spd X9 everything. Could I change the cassette, chain and shifter to 10 spd and would everything still work. Trying to work out what would be cheapest/most future proof.

I had the same issue but other way around, 9 speed setup and 10 speed mech. The answer I think is no. I asked the same question but for my needs and someone said it was the cable cam at the back that changes how much cable is pulled through, not just the shifter. I bit the bullet and bought a new 10 speed chain, cassette and shifter. If you could change the cam from your current to an X9 10 speed it would work (someone went from old X0 9 speed to new X9 10 speed, cams mounted on rivets in the same place so he swapped. My old X9 and new X9 were different mounting points)