Most of UI changes (not all) were introduced into the game when Community Test Environment was active. Community Test Environment is a separate client where DICE along with Community tested new features and updates before all these changes will be released for the retail version. I'm really sorry, but now it's impossible to revert all these changes. DICE team added a lot of UI customization options for players, so players can individually configure UI as they want.
UI menu was completely changed so players could easily switch between other Battlefield games, for example you can run Battlefield 1 from BF 4 if you have both games installed.
I completely understand you that you miss original UI, but we have what we have now and it can't be reverted. Since DICE is focused on BF V, they won't do anything for previous Battlefield games.