When you upgraded to K2 Five (5.1), deployed workflows were moved from their existing category folders to the All Items folder (root category) in the category tree in K2 Designer. A Known Issue article was released with a script to run that resolved this issue. Installing K2 Five (5.1) FP1 automatically runs the script and no further steps are required. No changes will be made if you have already run the script to resolve the issue. See Known Issue: Upgrading to K2 Five (5.1) or K2 Cloud (Update 4) moves deployed workflows to the root category for further information.