The process engine itself doesn't really care about process versioning directly, as far as it is concerned, two versions of the same process definition are concidered different processes (they just have the same name and a different version), and this probably makes sense as in reality two different versions of the same process definitions can actually be completely different.
You should always keep a version of your process somewhere if you still need it (for example if there are still process instances executing based on the "old" definition). So only if you make sure that all active instances of your old definition are either aborted or migrated to the newer version, you no longer need the old definition.
The process engine doesn't really care where those process definitions are stored, it just needs to be able to reload them (in case of restarting for example). So you can use file system, a repository (like Gunvor), a database, or whatever you prefer. If you use a simple file system, you probably want to have different files for each of the versions you're still using. If you use a SCM system, you can probably reuse the same file as long as you can then retrieve all the versions of the process you still need.
Kris