There's absolutely no way that I would condone this trade.
Firstly, I feel that making a trade this significant would considerably lower our chances at a title this season. Even if you say that our point guard and small forward positions are not significantly downgraded, our shooting guard position would be. Also, you can't underestimate the chemistry factor. These guys have been playing together for 4 years and keep getting better at playing with each other as a team. 4 months probably wouldn't be enough time to pull this unit together as a champion caliber squad.
Secondly, I understand how a trade like this extends our future some by trading a 33 year old small forward for a 26 year old one. 'Melo's still got his prime years ahead of him. However, there's no guarantee that he will fit in with the Celtic way like our beloved captain has grown to over the years. And, it's simply too mercenary a move for my liking. Paul's signed for three years beyond this one and I somewhat optimistically expect him to earn his money through his play and his presence for every one of those seasons. To just throw away a Celtic legend for someone who's a few years younger doesn't sit well with my sense of what's right. Plus, it would add a significant amount of pressure on Carmelo. It would be really easy for him to not be liked by the faithful and set up for failure and derision if the team didn't get a title very quickly.
So, nice effort on the concept, but a resounding no from me.