Dennis @ PerfectForms
My first suggestion is to never delete any fields from old versions. That way you can still report on old data even if you are no longer using those fields. Simply hide the fields.
If you are developing in another form any new workflow stages, behaviors, fields, etc… can be pasted into the production version. I would not paste or copy over existing fields. Even if the names are the same they will be treated as different fields in the DB.