Sure, there will *be* a new Apple Watch in the future. I don't think there will be a change in case design for at least 2 years, however. The Apple Watch is a new product and an entirely new category for Apple; They're not going to treat it exactly like they treat the iPhone. That's for sure.
Apple has some of the brightest marketers, designers, engineers, and tech-leaders in the world. You can bet that they're going to try their best to create the best new Watch they possibly can. You can *also* bet that they'll be supporting the first generation Apple Watch for many years to come. Apple is very loyal to their customer base in this way: they support their devices for at least ~5 years after their initial release.
Who knows, though? It wouldn't be hard for Apple to upgrade the internals of the first generation Apple Watch, either. Some people may "fall in love" with their Apple Watch and, when a new model comes out, they may want to stick with their current model's design. We already know Apple will swap batteries. Maybe they'll upgrade the S1 and the sensors, too. They certainly wouldn't have announced it yesterday even if they plan on doing it in the future.