Some parts of the UI are changeable, some are not. The ones that are not (at least, not very easily) are probably the ones most people want to change – the toolbar icons. You can use File/Options/Appearance to change some aspects of the program, ,and Documents/Change Icon for others. After that, you are developing your own theme, which is what user BadScribbler did here.
Scrivener 3 installs into its own folder under C:\Program Files, and you can keep Scrivener 1 and Scrivener 3 on your computer at the same time, so if you buy v3, you can still run v1, though the v1 license will not work with v3 – you will get a separate v3 license when you buy it.
V3 can convert your v1 projects to v3 format, but due to extensive changes in Scrivener, such as the Compile process, you will still have to do some manual tweaking of the converted project.
There is an upgrade guide here.