Originally Posted by themicah
It really depends on how far apart the outbound and return are.
If the return is more than two days later, then you shouldn't have a problem. Put your WP# in the record when you book the ticket. The miles should post 2 days after you fly the outbound. Then either change the FF# in the reservation at nwa.com, or simply wait until checkin and change it then.
In my experience (and I've tried several times), NW agents will refuse to change the FF# in your record once you've begun travel, regardless of how long ago.
That said,
rcs85551 is also correct (as usual!) in my experience, but I was trying to keep quiet about that loophole lest they close it.
The real issue is if you're NW elite, they may revoke your upgrades for downline segments if you try to change it. YMMV.