Okay, You're getting confused. A luxury ball doubles the happiness value of the pokemon when caught. So normally, a pokemon is caught with 70(?) happiness but if you caught it with a Luxury ball, it would be caught at 140 happiness. Trading a pokemon will reset the value to 0 regardless of any other factor. So in your case, the pokemon migrated to Diamond will have 0 Happiness.
Source:
Luxury Ball
Happiness