Currently, any updates to an active workflow will not affect those already on the workflow, only those new to the workflow. Ideally, you could choose to direct the existing contacts to the updated version. The only alternative at the moment is to remove contacts from the workflow and manually perform the tasks or allow them to go through the outdated workflow, which can be problematic, especially if there was an error with the initial workflow and you have a few hundred contacts going through the workflow... not ideal for the contact or the account.