A few thoughts here Michelle:
A/B Testing the email is typically the best way to test the email in these scenarios, in the future, I would recommend just creating an A/B test as opposed to swapping the emails entirely.
Another thing to mention is it's a best practice to build asset specific emails under a parent asset program that tracks membership. From there, you can add the asset program to the stream and Marketo will be smart enough to not send the asset email to anyone that is already a member of the parent program.
Since you are where you are now, you could always just add a filter to only show emails sent to people that were created after you made the change. This will remove all of the older people and may give you a better understanding of performance compared to the past emails.
Hope this helps. Let me know if you have questions regarding anything I wrote here.