I recently had to change my PC and instead of reloading everything I only loaded X2A upgrade. By not loading my 7 PE, 8.5 PE upgrade and X1 expanded upgrade, am I limiting what my X2 upgrade does? If so, how far back should I re-load and would I have to un-install X2 and start from 'top - bottom' again?

Versions (8.5, X1, X2, etc) stand alone, but you usually need to install all of the patches or sub-versions (X2A) to address the bugs.  I used a clean install of X2 to identify and get rid of all my old, unsupported 32 bit plugs and other 'junk' that I had loaded to test but never used.
 
Glen