Good questions, the upsell program is from Coach into Business Elite, I've done it several times. The strange thing is that this last time I was able to upgrade from an L fare on a Delta ticket and on Delta metal from coach to business elite. I specifically asked the agent if it mattered if it was Delta stock or Delta metal and they said "no". If there is a seat in business elite then they can upsell me to Business Elite.

The best part is, you can even use your Delta one way upgrade certificates to bump up so you don't loose miles.

As an example, I was on a $1,000 round trip ticket and on the return leg AMS - JFK paid 300Euro to upgrade on a T fare from Coach to Business Elite and have done this several times.