I got an email form Seabourn about this several days ago. This is my understanding of the changes:

Cruises up to 13 days-1 formal optional evening
Cruises 14 to 20 days-2 formal optional evenings
Cruises 21 or more-3 formal optional evenings

The default dress code after 6:00pm is usually elegant casual (jacket but no tie required). The biggest change with the new policy (which the email said takes effect with October Cruises) is that under the new policy, you WILL be able to eat in the main dining room on formal optional nights if you are wearing a jacket or blazer with collared shirt, even WITHOUT a tie! (The old policy required you to wear a tux or dark suit/jacket and tie in order to eat in the main dining room on Formal Optional evenings.)

Hope this helps! It is VERY comfusing!