OK, here's the scoop.

I called the Silver Elite line, ended up w/ nwa.com customer service. She told me to modify it through the web site (duh). I tried it, selecting Y for the new flight - $406 due. She recommended i book it and discuss it with someone at the airport. Sounds like a good plan to me - not.

I said thanks and hung up and canceled the transaction. Went through the web exercise again, selecting F instead of Y (original flight was P). Lo and behold, total cost to change ticket $0.01. So I put in all my CC info for a one penny charge, and all is good, as long as I can figure out how to pad my expense report for the extra cost .