The risk will largely depend on what modules you already have installed and whether you are running with any source code in place on the site (as opposed to all code-behind already compiled into DLLs -- i.e., "PA" versions of modules).  Any source code present will be re-compiled under the new framework.  When this happens you *may* encounter some incompatible source (and, thus, errors).  I ran into this very thing recently but , thankfully,only with a single line of code on a site with many custom-written modules.  In that particular case the DNN core code and modules did not have any problems going from 2.0 to 3.5. 

You'll definitely want to try the upgrade to the 3.5 framework on a test copy of your site before attempting on the live site.
 
Good luck!
-mamlin