Updated June 10th, 2019.
If you buy a Mac, and Apple updates that model the next week, bring yours to an Apple store and trade it in for the new one (plus any money difference). You have 14 days to do this. Or just bring it back, and get a refund. You have fourteen days, no questions asked. Even if the box is opened. That goes for almost everything Apple sells.
Here’s the complete policy, on Apple’s website.