Im looking at a bike with new 3×10 set up. I want 2×10 and may go 1×10 in the future. Questions,
I believe I need to swap crank set from 3 to 2 as BCD are different?. Is this right?
Can I keep a long cage rear mech? Or should it be medium.
Will I need a new front dérailleur? For 2x 10
Will I need new shifter? Currently 3x.
I,m wondering if I could use same shifter and front mech ? Any input greatly appreciated. Thanks.

Only thing I did was take off the large chainring and replaced it with a chain device

1. BCD is the same although chain line is (I believe) different. Also a 2x chain set is prettier.
2. Rear mech is fine.
3. Front mech is different and may not work well with full suss as the drop required is shorter on 2x so having repositioned the mech May touch the chain stay.
4. There is a switch on the front shifter for 2x.

Wanting to do the same thing *waits patiently for answer*

Depends.
You can whack the big ring off and fit shorter bolts or a bash and limit down the FD – will work if possibly not exploring full potential of 2×10.
You can fit a 2×10 specific crankset and make use of better spacing, wider ratios and wildly reduced cross gearing due to better chainline. This can still be done with a limited Triple FD and shifter or you can buy a new 2×10 shifter and mech. One is obviously cheaper, the new mech has an advantage in having a shorter cage though which will help with chainline rub.
Up to you and what you want to spend really.
My Chameleon is all SRAM 2×10 and my Spicy is SLX 9 spd M665 double and bash with 10 spd XT cassette, M785 front and rear mechs (front is 2x specific) and SLX 3×10 spd shifters and works beautifully if not quite so good at cross gearing as the Chameleon.

Sell 3×10 cranks
Buy 2×10 cranks
Fit cranks
Bodge front mech with hi/lo stops ( there will be one dead shift on the front shifter, but it doesn’t matter.)
Ride

Simples!

In my experience there should not be a dead stop on the front shifter that you would notice, it just only fires twice due to cable tension.

Thanks all. 2×10 will be diff ratio poss 28. / 39. Or similar.
The front mech and shifter xt were puzzling me. Ideally dont want to replace but want it right.
Thanks