couple of questions to future proof my knowledge

[mikgit]

OK So my FS bike has a press fit BB with XT 2x(10) cranks. On the assumption that some time down the track I replace the drivetrain with something else, my other bikes all have threaded external BB's, is it a case of just pulling the cranks and slotting them into an external BB bike? IE same crank/spindle no matter the BB, or BB specific.
Continuing on I have some AMClassic wheels from 2012, currently running as 9sp, will 10sp just slot straight on? What about 11sp. I know my 11sp road wheels came with a spacer for 10sp, but thats road. Do I need different freebodies or anything?
Oh and assuming everything is shimano.

[Duck!]

There's no such thing as future proofing, because They will keep moving the goalposts....
BB92 press-fit is the same assembled width as external bearing threaded, so yes, the same cranks will swap between frames & BBs. Additionally, there are adaptors that will enable the crank to be fitted to a BB/PF30 frame should you find yourself going that way down the track.

10 and 11-sp. cassettes will fit straight on the 8/9-sp. hub. As with road, 10-sp. is basically the same overall width as 8 & 9-sp, the sprockets are just packed in closer to make the extra one fit. 11-sp. is wider at the sprockets, but the enormity of the sprockets on offer enables the large one to be set back off the central spider without fouling the spokes, so the cassette core is the same width as 8/9/10-sp.

You will need an appropriate rear derailleur if changing to 10 or 11-sp. Shimano began mucking around with cable pull ratios from 10-sp. onward, so a 8/9-sp. RD will not play with a 10 or 11-sp. shifter. 11-sp. is different again, so will also not mix with others.

[mikgit]

Cheers, good to know..

I only mean future proofing with what I already head, moving to other stuff I already have.
More of if I go 1x XT on my Scott spark, I'll move all the 2x10 XT over to my Zaskar reissue, and all the 3x9 XT from that to my Yeti ARC, but keep the wheels the same... but not doing any of that any time soon, unless I decide to sell m Xizang, then things will get rowdy.