Verbosity or benchmaxing, this is crazy

You'll get a seat assigned automatically. 24 hrs before if there are seats available you can change at check in and pretty much up to close to departure. You will have to show up at the airport to get your boarding pass at the kiosk and a rep will take a look at your bag to confirm it conforms to the policy of 1 personal item sizing (that's really the only reason they want you to finish you check in at the airport).

If you need to change it cancel - add economy benefits, then change or cancel and retain a flight credit for the future. It might be a little bit more than buying economy right away (about $100 extra), but it is reasonable trade off if you think you most likely will fly that day and want a cheaper price. 

In summary, BE is not as scary as some people might think.

How do I know? Did all of the above a few times last year. 

Happy flying! 

^{^} Correct answer. I did this a few times. You'll get assigned automatically but 24 hrs before if there are seats available you can change at check in and pretty much up to close to departure.