With regards to the new music, the old music is still there, of course, but there's new music to bolster it.
There are 3 new tracks per faction (so 18 in total there,) a new main menu theme, and around 4 or 5 new battle themes. As for sounds, there's a bunch of new sounds for all of the neat new abilities on the ships and that each faction have, Wail of the Sacrificed, an Advent Rebel ability, has a particularly creepy sound effect when you use it.
As for the textures, in a video that was posted where they showed off the new shadowing, it was mentioned that they touched up the textures.
Video is here: https://forums.sinsofasolarempire.com/418857
The bare bones of the UI(which is fine anyway) is the same, all the buttons are in the same places, etc, what's different is the graphical style, each race has their own style, for example, here's two screenshots of me in games as TEC and then Advent: