I e-mailed them once wanting to do this, being Elite on DL, but a NW ticket and this was their response:

Dear Mr. Poor,

Thank you for your e-mail to Delta Air Lines.

Since your segments are ticketed as Northwest Airlines flights,
operated by Delta Air Lines using our aircraft, you will not be able to
view your flights itinerary on delta.com .

Since your are traveling on a Delta aircraft, and your flights have
been ticketed as Northwest flights, you will not be able to upgrade
using your complimentary mileage upgrades.


If you have additional questions, please contact Delta's Special Member
Services Desk at 800-325-6330. Our agents will be glad to assist you.

We appreciate your business and look forward to seeing you on your next
Delta flight.

Sincerely,

John
Online Customer Support Desk
http://www.delta.com


So if you have any insight how to request an upgrade, let me know.