What's the worst about this change is that low level seats tend to drop in those last 72 hours. It has been a huge perk for PM/DM to change for free. For example, last year I was able to change CDG-IAD//DCA-LGA to CDG-JFK the day prior to departure for no fee when the CDG-JFK low level seats dropped. Now this would be impossible. I was going to have to really push to renew PM this year and the award change fee waiver was one of the biggest perks since you could usually do great things a day or two prior to departure when seats opened up. Probably will settle for GM now.
I think that Delta will say the reason for this change is so that unused award seats don't fly empty, however, I think the real reason for this change is to lock passengers into using more miles since the low level seats tend to drop at the last minute.