There isn't any out of the box method of doing this, nor I am aware of any tool to do so either.
*At your own risk you could try adding the workflow to a solution and exporting it, then manually editing the customizations.xml file and updating the PrimaryEntity element and then editing any references in the .xaml file(s) in the Workflows folder.
You'd need then to delete the existing workflow and re-import the solution.
I'd test this in a different environment before doing it in production just to be sure there are no side effects.