It's not as if these 62 A320s are used exclusively on short stages. Thye can call it a test but two months is a very quick roll-out. From the Skift link:
Delta is making the change only on its Airbus A320s, now with 157 seats. The first airplane will start flying this weekend, with the rest coming within the next two months. The airline calls the program a test, but it’s open-ended.
Seats in economy class, including extra-legroom seats, will recline two inches, down from four. In first class, seats will go from more than five inches of recline to roughly three and a half.