The only way is to re-create them in Rule 5.x. However, there is no reason to in general (though the use case you allude to does raise a point, though one that can be worked around). There are lots of posts on this; here is one:
Editing the export (not necessarily valid JSON, by the way--just a similar format) is completely unsupported, and I would not recommend that path.
I'm guessing Rule 4.x was never modified to include Rule 5.x rules in the pause/resume list--understandable given that it is older, I suppose. The reverse, as you've discovered, does work. If this is a need you have, perhaps you could consider another workaround, like a conditional with a virtual device, or a hub variable with a connector and a 4.x rule that reads changes in the connector and pauses/resumes the "real" 4.x rule.