You probably will not be able to upgrade unless you want to pay for it. Delta flights (probably all airlines) have been very full in recent months and courtesy upgrades non-existent except perhaps for the highest level frequent flyers.
I would suggest using the Delta website to check your agenda and seating assignments (you need a frequent flyer number and PIN). You can change your seat assignments online to any seat that is showing as being available. Avoid last row seats and seats in front of emergency exits as they will not recline. With a late night flight, you will WANT to recline.
Consider before you go if you are willing to be bumped for compensation. This is happening a lot recently and you are traveling during a very busy season. Some people are happy to be bumped and collect the compensation. When I flew recently, they were offering $200 towards future travel if they got you there within 4 hours of the original time and $400 if it was later than that. However, they warned that it could even mean staying overnight and flying the next day. I am ASSUMING they would pay for the hotel room as well but I'd confirm that before I took the offer.
Happy anniversary!
Peggy