Currently, JIRA doesn't allow to edit an active worflow though the GUI. Although, if one modify the URL passed to JIRA and supply a customised URL e.g.

.../DeleteWorkflowTransitions!default.jspa?workflowName=test&workflowStep=6

where test is the active workflow, he/she will be able to modify the worflow.

Since a given step is deleted, Issues which are in the deleted step can't be transiton to any other step and the Integrity Checker will shows invalid statuses.