Once you have your workflow going and a need arises to change your workflow process, you cannot simply update the workflow, since it might disrupt the current assignments. Either you can wait till ALL the items in the workflow are signed off, OR you use the “Save as new version” feature.
“Save as new version” allows you to create a new version of the workflow. You can make modifications/process changes to this new version and save it.
When you are ready to release the new version of the workflow, simply set the “start date” on the new version of the workflow to a date when you want to start using it.
This will automatically update the “end date” on the previous version of the workflow.
All the current assignments will still continue using the old workflow. But any new submits, will use the new workflow version.