You can even delete the template helpers now; the statistics for them will be retained.
In order to keep statistics history for an old entity separate from a new entity you just need to ensure that they get different entity IDs.
Creating template helpers was a way to guard that entity ID so that nothing else could take it. There are a few other ways to accomplish the same goal but this is actually a pretty clean and easy way to do that.
Thanks for updating the post with your solution.