Updating a workflow using NWAdmin, versioning?

Hi,

 I am trying to update a workflow using NWAdmin.exe. To do that I run it with the "-overwrite" parameter. The problem is that my "old" pending tasks using an old version of the workflow will fail (because they are not using the same variables). How to use the old version of the workflow for these pending tasks and update to the new one?

 Thanks!

Thibaut