Has it been everyone's experience that redeeming miles for a mag you are already subscribed to(like say, The Economist) will extend the subscription?
I have renewed my existing Economist subscription three separate times using miles with no difficulty. The last two were even using someone else's frequent flyer miles, as you have to option, at least with Northwest, to use miles for a gift subscription. In each case, after a few weeks, the expiration date on the address label incremented by a year.
AGS