AA is much better about this. I just used miles from my Aadvantage account to upgrade my wife on a domestic itin with an "S" fare, which is about as low as you can go on AA. Neither of us are elites with AA. And I was able to do this more than two weeks before the flight.

I assume CO and NW are more restrictive to make sure more seats stay available for elite upgrades. As a NW elite and an AA non-elite that sounds good to me.