Some guidelines:
1) As said by Rick, always use the "Generate version" functionality in the WF builder before you do changes. This way you will be able to find the old version and even revert to that if necessary
2) Even if you create a new version, be VERY careful when you decide to do any changes to the existing tasks. Tasks do not have versions.
3) The running workflow instances (the ones that have been started before your changes but are still in process when you transport your changes to test/production) continue to use the old version. This is exactly why you must be careful (for example with the tasks).
4) In some cases (when you are doing really big changes) it is even advisable to copy the existing workflow and do the changes to to the copy and then start using it (of course you need to do the relevant changes to the workflows starting logic also).
Regards,
Karri