From what I gather from the wording, is that the UI layout, design, etc isn't changing. Just the bit of software that runs it. It's like if you took the dashboard out of a broken down 2005 Honda Civic and put it in one that's only driven 10 miles. The thing the dashboard is connected to has changed, but the gauges all look and are used the same way. Plus the overall driving experience is probably better for it. There's probably a better analogy out there, sorry. If you use a custom UI, it depends. People have asked if the new engine will read the same UI files (XML), with no dev response. If the engine does indeed read the same files, then the previous paragraph holds. Otherwise, you'll have to hope the person who made your custom UI is still around to make a new version of it.