Answer received from NWA less than definitive as well.

Thank you for contacting nwa.com Customer Service.

Confirmed flight changes will be offered during self-service check-in
for reservations that allow flight changes. Select the "Change Flight"
button to view alternate flight options.

FlyNow allows customers to confirm alternate domestic flights for a fee
of USD 25.00. FlyNow options are available all day on the same day as
the originally confirmed flight(s). FlyNow options are only offered when
check-in occurs on the same date as the confirmed flights.

If FlyNow is not available, or for flights the next day, a change flight
option may be available that may include an additional collection for
any fare difference along with applicable fees.

If you have additional questions, please visit our Frequently Asked
Questions section at http://www.nwa.com/checkin/faq.shtml

I believe this information will answer your questions about this
reservation.

Thank you for choosing Northwest Airlines. We value your patronage and
consider it a privilege to serve your travel needs.

Sincerely,

BobbySue
nwa.com Customer Service


Message:
I have a question re: the "Fly-now" option. Do you need to keep the
same routing,
i.e.--if you are originally ticketed for a fsd-msp-dtw-ewr routing can
you obtain the
"fly now" option for just fsd-msp-ewr (eliminating a connecting flight)?
Also when
does the "fly now" option become available--12 am Eastern time or
Central time on
the day of travel?