Based on my redemption inquiries, the above link's statistics are not accurate. This is based on redemptions mostly on NW, CO, DL, US, and fewer redemptions on UA, LH.
Northwest is less harsh than CO. You can change awards free if you use nwa.com and do it 30 days in advance. If it's less than 30 days, it's half price if one makes the change on the internet.
Other airlines beat even NW. They are free.