Normally, I avoid posting something I'm not fairly sure about, but there're flenty of FT folk who will correct me. I know NOTHING about whether or not a NW Plat would be treated differently, but from the email cert NW sent me, it looks like you could change the ticket, but would incur a $100 change fee:
"Changes/Refunds: Once issued, free tickets have no cash value and are
nonrefundable. Changes may be made prior to scheduled flights for $100
administrative service fee. However, any change must meet the conditions of this
program. Changes which are requested after the originally scheduled/ticketed
flight are not permitted and the ticket has no value and will not be replaced.
Ticket will not be replaced if lost or stolen."
Now, what is NOT said is if the ticket has to be exchanged for another K fare ticket...but I'm pretty sure it does. If this is so, you might have difficulty finding one closer to your date of travel. I know I am having a devil of a time finding anything <6 weeks out!
Anyway, good luck!