Actually, the classic openings are sometimes interchangeable. If the opponent doesn't see what you're doing, it would be fairly easy to transform a Ruy Lopez into a King's Indian Defence with a developed Bishop.
If he does, then you counter, he counters back, and before you know it, you're in the middle game.
That is exactly how chess game should be played: Knowing lots of tricks under your sleeves which your opponent hardly see --forcefull moves that push your opponent to commit mistakes but which moves seem right from his vantage point. But it is easier said than done because the problem is that chess variations are so numerous and so complicated that we may know some lines but the hardest and most important ones we know not, and that's where the problem lies. Not anyone could identify which variations to employ in a game, or which variations run against the established ones plus the ever changing fabric of the game, etc., etc.
Virtually all of us players are particularly concerned about our Opening Moves. Although some are knowledgeable enough, others play opening moves just to answer back from the just concluded move by an opponent. Opening moves are not only for the sake of moving your first chess piece but it is also the start of a certain line opening or course the chess player will be taking. The huge mistake majority of chess players make is when a certain chess opening becomes irrelevant with the line variation the player employs. It can be likened to a tall tree whose fruits are hanging from the top branches and twigs and to be able to get them you need to climb starting from its trunk and several branches until you reach the twigs where the fruits are connected. Taking the wrong course or branches often leads to no fruits gathered at all and you have to change branch to be able to get the fruits. But in chess game, taking the wrong line variation from the established opening move more often than not leads to defeats. That is why the established opening moves after several follow through should be supportive of the chosen line variation and not all mixed up say, Queen's Gambit or Ruy Lopez becomes new variation when, depending on your opponents line of variation changes the fabric of the game -- you must not veer away from the right combinations of variation or else it would be disastrous on your part.