You will "up" your strength on the dumbell presses, and maybe have a little loss of strength at the standard Bench press when you return to his exercise after a break, because you loose a bit the "handling" of the barbell. This loss is not a real loss in muscle strength, but it`s a loss of optimal coordination for the bench press. So it`s only temporarly. When you come back to the bench press, you will notice the strength in the first 3-5 workouts will always go up, because the body learns again to push the given weight more efficient.
I´m always changing my first exercise after 4-8 weeks.