OK, apparantly I messed up, not having done much of my own repair. I went to resac an Esterbrook J and found the section stuck too tight to budge. So I dipped it in warm water. Then in fairly hot water. It finally came loose.

But when I went to put the pen back together later, the barrel had expanded and was now way too loose on the section. And the screw-on cap was now a friction fit --fairly tight! (That didn't happen on the other pen!)

Perhaps water is not the best medium for heating a section/barrel before removing. But it seems this one needed a bit more heat than normal since it was stuck on so tightly.

I suppose my Esterbrook body is ruined -- can't be reshrunk?
(I tried the freezer for 15 minutes).

Other options in the future? I do this so seldom that a heat gun seems overkill/expensive. Maybe the hot water idea was OK, and I just left it in a couple seconds too long?

Hot water does not give you sufficient control.
In the old days, they used an alcohol lamp. I use a heat gun. You only have to ruin a couple of pens and you have lost more than the price of the heat gun.
I don't think your barrel can easily be reshrunk.
If it is any consolation, we have all ruined pens while learning how to repair them (and often more valuable pens than an Estie!)