The Upgrade Management Tool will migrate the reports from an older version of BusinessObjects to a new one. It only updates some of the report properties in the CMS database - it does NOT update the report files. That's ok, though, because reports written in Crystal 2008 will run in BO 4.x without any problems.
So, you do NOT have to actually upgrade the reports themselves to Crystal 2013 as part of your server upgrade. You can upgrade them as they need changes/fixes by opening them in Crystal 2013 and then saving them.
-Dell