We've never tested that situation. The major version was changed from v4 to v5 to indicate that it isn't backwards compatible. Compatible versions increment as minor versions, like v4.x.
The easiest thing to do, is to put the old course back and launch a new course in v5.
Otherwise, I can't think of an easy way to automatically complete any course without overwriting the score and/or causing other restoration issues without seeing it in practice.