Posted above: ... simply use on-line check-in and change your # there...

Well, I tried that too before arriving at the airport, and the system's response was that I could not check in on line and would need to do so at the airport. But when I tried using on line check in a few minutes later without trying to switch my FF acct, it let me check in on line. Earlier in the year, I credited numerous DL flights to NW expecting to build NW status later this year via switch at the gate when flying on NW, but now I am stuck with requalifying on CO because I'd rather keep the U/G and have been scoring U/G almost 100% of the time on NW (much less on CO).