I think I can honestly say nothing. I'm not a fanatical club swopper / changer and every club I've bought has improved my bag. The only thing I'd say is I used to have a Titleist 816 H1 21 degree hybrid which I used to hit so well but I got rid because I just didn't have room for it in my bag when I first bought my JPX irons a few years ago.
I'm in a position now as well where I've got a couple of clubs at the top of my bag which do very similar things but I love both of them and can't decided which one to keep in my bag.
Not something I've sold but my Ping i25 Driver, i smashed it into the hard driving range floor at Dubai Creek GC - turns out almost impossible/impractical to swap shafts as it needs and adapter and a bits and pieces.
I will forever remember the coach who told me to try a new stance then first time out I did that, it was a great driver
I never have too much remorse or regret. If I've moved clubs on its because I believe what I was getting was going to help. Some did and some didn't. That's the gamble of buying new gear. Those Cally fairway woods though were simply the best ever and I've never found anything to match them