You can only legally upgrade to the latest release which would be AutoCAD 2017.  And technically speaking it would not be an upgrade as there is no path for upgrading from 2006 all the way to 2017.  You basically buy a new license to install/run 2017.  There is also a yearly subscription fee which can run a few hundred dollars or more depending where you are located in the world.  Any AutoCAD product that came after AutoCAD 2006, from 2007 all the way up to 2017, could conceivably open the drawings that were created with AutoCAD 2006. 


Edited by John Connor - 22.Feb.2017 at 19:25

Generally speaking you should be OK.  There is the "Ribbon" to contend with as the Dashboard has gone the way of the dodo bird.  There are toolbars and tool palettes now but you can still access the drop-down menus if you want.  You may feel lost for a couple of days but that should be the extent of it.  If you remember command aliases like "L" for Line and "M" for Move then you should be fine.