I don't think the HomePod sees any update until late 2019 at the earliest, and likely longer than that. It's Siri that will be changing over time and as long as the current HomePod is capable of being remotely updated I doubt the basic design will change. It is possible that rumors of a cheaper "Beats" branded speaker with Siri could surface this year, but obviously it won't sound as good if that happens.
Now one thing I think *could* happen this fall? No rumors of this at all but I *could* see Apple dropping the price from $349 to $299 to help sell more of them.