HTC didn't change a whole lot with the HTC One M9, they merely refined and fine tuned some of the already recognizable aspects of the phone. Like the overall design for example. The metal unibody design is present, just as it was on the previous HTC One (M8) and the original HTC One M7. That doesn't mean HTC didn't introduce some new things to their latest flagship. There is a new processor here of course, as the One M9 will be powered by the brand new Qualcomm Snapdragon 810 octa-core processor. It also comes with a brand new camera that sports a much higher MP count in the sensor, sitting at 20MP for the rear camera and the 4MP UltraPixel camera from the One M8 has been placed in the front.
HTC's new changes aren't limited to just hardware and design though, with the HTC One M9 they're introducing a brand new way to customize the HTC experience with a theme engine, which will afford users the luxury of picking from a preset list of themes both from HTC and from developers once things get up and running. Users will be familiar with theme engines from their implementation throughout CyanogenMod and Paranoid Android ROM's, and Sony employs this characteristic in their more recent Xperia line of devices as well, so it's safe to say that HTC was inspired by the use of the theme engine from these sources as well as others like Xiaomi's MIUI ROM which has a massive theme engine and theme selection. Themes are new to HTC devices though, so it will feel like new to HTC loyalists who tend to keep things stock.
Excluding the theme engines from specific ROM's, HTC's upcoming theme engine actually aims to be a little more robust than what's already offered on other stock Android experiences. They will end up having a theme store with the pre-designed custom themes from the community, as well as HTC themselves. HTC also aims to give a little bit of creative power to the user, by letting them almost dictate the theme creation with their photos. The way HTC's theme engine works, cropped photos are used to select all the elements of the theme which includes icon packs, textures, the wallpaper and even the font, so the user has a completely unique theme based off of their photo selections.