That's a really good question. I use responsive elements in development now so I don't really think about virtual resolution since vector graphics scale to whatever size the screen is. Things that are pixel dependent such as photos and videos however would need some adjustment most likely.
The iPad Pro has the same pixel density and UI scaling factor (@2x), so the size of most icons and UI components is the same, but the surface is obviously larger. However, the iPad Pro does require larger app icons for the home screen (link). Otherwise, modern iOS applications compiled with size classes or universal layouts should work without modification.