Currently have a Specialized Sirrus that has a 3x8 drivetrain with a squaretaper bottom bracket. Looking to switch to a 2x10 set up and have found a 2x CX crank I'd like to use that also supports a square taper BB. My question is do I need to get a new BB or can I just swap out the crank/crankarm and be good to go? Obviously I know I have swap everything else over to 2x10 components.

Square taper BBs come in different spindle lengths, and you need to buy one that is the right length and taper, in addition to fitting your frame. Most likely you will have to buy a new BB, but sometimes you don't have to.

BBs are cheap; pick out a crankset you would like and if the BB you have fits, great, otherwise just get one that does. Basing your crankset choice on the BB you have is letting the tail wag the dog.

I don't get it. You appear to be willing to change out many other components yet you worry about a bottom bracket? The crank you buy will decide the bottom bracket you need. Filter out the noise. when you decide on a crank, its specifications will determine what bottom bracket you need. If the current one works, so be it, but most triple cranks require a different bottom bracket than double cranks

What is the drive train?

A lot of MTB stuff is compatible speedswise.

Not sure why Phil feels the need to act tough.