Workflow version control
When you import a list or site workflow, be aware that any existing workflow of the same name will be overwritten. The overwritten workflow is not retained and cannot be retrieved by default in Nintex for Office 365.
Best practice
To manage workflow versions and to be able to recover an overwritten workflow, Nintex recommends keeping the exported copies of all production workflows in dedicated document libraries. By exporting and storing workflows into a document library (with versioning enabled), all previous versions of the workflows as well as metadata describing the changes between versions can be achieved. This best practice allows for the recovery of any overwritten workflow or restoring a workflow to a previous version.