if (big if) I went to double, have decided I’d want to go 10 speed for broader spread of gears – probably 36 / 24 rings and 11 / 36 cassette

what would I need to convert existing 9 speed?

rear shifter – would need new
chain – new (obviously)
cassette – new
rear mech – assume existing will work?
jockey wheels – new?
chain-rings – will 10 speed chainrings fit on 9 speed XT cranks? (doubt it)

if existing cranks would work, guess it could make sense when chain and cassette wear out

thanks

Just get a middleburn duo 27/40 and stay with an 11-34 cassette, seems fine to me.

You could keep your low gears and stay 9sp with an SLX 36/22.
Keeps the expense down in case you decide that you prefer the bigger range of the triple after all.(We have had several riders on a 2×10 demo bike who decided they preferred a triple.)

Vaguely remember reading somewhere that a 10 speed rear mech is required, though I suspect that might have been Shimano/SRAM marketing speak.

Can’t believe 10 speed chainrings wouldn’t fit on your current chainset. Though looking at the price of chainrings buying a new chainset might be just as cheap….

But I’d go for the 22/36 suggestion and keep the current setup. In fact that probably just means a 36t chainring…

A 9 speed Shimano mech won’t work with a 10 speed set up. It has a different cable pull ratio and a different parallelogram action.

Practically speaking you will get away with the cassette of course, a chain to match and the shifters. Assuming not too much brand mixing the front mech will work with anything. The chainset will cope fine with the chain and so will a rear mech. I run several versions, all of them bastards, including 1 which is ancient 5/6 speed Campag Super Record mechs, 10 speed Campag shifters, 10 speed shimano cassette a Connex chain and a Stronglisght chainset. Faultless and silent.

The 9 speed rear mech DOES NOT WORK with 10 speed. The cable pull is different so the indexing goes out and you won’t get all the gears. It’s like using a Shimano mech with SRAM numbered shifters.