We need to add to our frontcourt not swap.
If we can upgrade our D in the post and help defense, I would consider it and I disagree with you.
Sully is a good player on the offensive end and he can rebound. But his D is subpar. KO is a good passer and decent offensive player but he can't play a lick of D. As help defense goes these guys are terrible. You can only have one porous defender on a good team and they both can't be in the frontcourt.
Sully has a bit more up side left, but not much on the defensive end.
I disagree and think that he has no upside left. Good player in many aspects already but almost the same player he came into the league as a rookie. Guy was skilled and from the get go, his three point shooting is worse, shot selection needs much to be desired. But a deft rebounder and post scorer and not bad at mid range. The area he could improve most in, his body, is something he has shown zero desire to do so.
I don't mind upgrading PF, but then you're suddenly asking for an awful lot. Can the Celtics add a stud Center and still find a PF better than Sully/KO in order to replace one of them? That won't be easy and or will take a lot of time.
I'd prefer to get a do everything center, especially defense but also offense. If the Celtics can get a really good center then Sully is easily good enough to be the Celtics starting PF going forward. And KO is easily good enough to be a backup (unless he fell off the edge confidence wise or something). We'd still have Zeller too so if you needed defense he's there. Kelly would still have to earn his minutes with good play even as a backup, but I think he could do that with no problem.
Then, what if the center we get is purely a defensive stud? Then we'll absolutely need both Sully and KO or someone else who can put the ball in the hole.
I'd love an all star at both center and power forward but an all star center will solve most of the Celtics big man issues all by himself.