To clarify previous responses, the Delta complimentary upgrade program does not cover long haul international flights like flights to Asia/Europe, etc. See below for covered routes --

http://www.delta.com/content/www/en_...-upgrades.html

In rare cases, you may get an operational upgrade ("op up") on these long haul flights when coach is oversold and they need to make more coach seats available. Just because there are open seats in business does not mean you will get it for free. They can and will fly with empty business seats.