Indeed, now only W and S fares can be upgraded to business (O class). DL does not provide the booking class for premium economy fares, although if you select the "W class or higher" fares, then you are surely going to have a W or S (or higher) fare.

Upgrades can not be processed online. You have to do that with an informed DL agent. Many don't know the rules well, so insist on the W or S booking class, regardless of what they say.