If you want to use the Reassign workflow functionality in the Web Client in an upgraded environment, you must configure your workflows. Your existing configuration in the Desktop Client for the Reassign functionality will not automatically appear in the Web Client.
If you want to see Reassign in the Web Client, you must relate the applicable Workflow Step Class to the delivered WFMultiStepReassign method, or to your own custom methods that use the MultiStepReAssign API.
For more information, see MultiStepReAssign client API and its delivered methods.