New hires today don’t need to be worried about the 321’s and Trans-Atlantic flying. You’ve still got a long time before even the first gets delivered. Then, it’s not like they’ll all just show up at once, they’ll get delivered over months/years.
Every plane that gets delivered will start on domestic proving runs.
At a minimum, every CA will need to do a supervised Atlantic leg before being qualified.
That means LCAs will need to get qualified first.
They’ll almost certainly fly all of the Atlantic flying out of a single base (DCA since it’s smaller?) at first.

My point is: It will be a long road before FO bidding on the Airbus is affected by that flying in a “normal” way.