I noticed that DL is withholding more and more seats for pre-assignment. So in general, you might just be fine.
Not having a seat assignment does not give you a better chance of an operational upgrade - it's done by status and fare, and from what I have seen, they do not mind to switch seats around to insure the right people are being upgraded.
Assuming you are on DL80 on 04/18, itn.net shows J7 D7 I7 Y7 B7 M7 H7 Q5 K6 L5 U2 - so the chance the plane will be oversold is very slim from my experience.
And I just saw one empty seat in 25B... good luck!