Triple chainset to single chainset; BB compatability? I'd like to upgrade the chainset on my GT Avalanche 1.0 2013 so I have no front mech or shifter and just one ring on the front; just 10 gears instead of 30.

My current chainset is a Shimano M522 Octalink triple with a Shimano es52 BB and a Shimano HG81 10spd cassette and I'd like to upgrade to a Shimano Zee 10sp M640 with an m980 xtr bottom bracket (or similar), so essentially just 10 gears instead of 30. I believe the shell on the bike is 68mm. 

In my case, I understand there is a difference between the BBs, is it  just a straight swap of the BBs and disconnect of the front mech/shifter, assuming I have the correct tools? 

Would I also have to change my cassette and chain? Maybe even the rear shifter and mech?
Yes, this will be a straight swap (although you will need a cartridge BB tool, crank remover and HollowTech II BB tools)

The M980 bottom bracket is compatible with 68/73mm English threaded frames and HollowTech II cranksets such as the M640.

You would not have to change anything else on the drive train unless they were worn as running worn chain on a new chain ring can accelerate wear. However there are other things:

You will need to consider a narrow/wide chain ring or some form of chain retention to avoid chain drops once you remove the FD. As the Avalanche doesn't have ISCG tabs an option would be similar to this guide from Bionicon.

You will need to shorten your chain by removing links to fit the new set up. Use a chain equation such as this from Park Tools to get the correct length.

Your current set up will also be running a long cage derailleur which will now be redundant and you could move to a medium length RD. The longer cage could exacerbate chain slap so may be worth changing if that is important too you.