The Tavern

I was wearing a bad +0 cursed helmet when Ash offered a curse I rather liked and I had a +2 hat of willpower I wanted to put the curse on
So I shattered the helmet, tried to curse the hat - and discovered that Ash didn't offer the nice curse any more.

With hindsight I understand I was warned about this ("Ash will withdraw [sight and] a curse" or something like that)

Is there a way I could have gotten the nice curse on my hat? How do you switch equipment/curse with Ash?

I've had this happen too. I imagine you're not supposed to; that is, you're supposed to only shatter when the new equipment is better enough than the old that you're willing to wait a curse timeout with it uncursed in order to equip it. (Which means that you're fully cursed then currently-offered curses are inaccessible, I assume?)

Yeah there is no way, it's intended to be that way. You have to shatter your current equipment, equip the new item and wait to see what curse generates (possibly rejecting it to wait for a different result), there's no way to farm for a preferred curse before you shatter your old equipment.