Just tell your boss a good opportunity came up and you decide to take it, it
is none of his/her business, in the first place. Your company can assign
you to any geographical location just like you have every option to take it
or not. Your employment is totally at will, this means Company can terminate
you for any reason (not your fault) or you can fire them for any reason.
Don't be fooled by other folks' "Burn Bridge" -- there is no Bridge, the
only Bridge is that your capacity to perform a professional duty that is
market worthy, if you have it you will always have option, if you don't, no
body will shed mercy on you. This sounds cold, but it is the truth. So be
clear-minded and be honest to yourself.