I thought I had posted to this thread ages ago but guess not.
I've upgraded some dive gear but only really regret buying things that have never been used I found out quickly just weren't required or the right tool such as:
My collection of high volume masks.
A couple of impressive looking knives that now live in the spares tub. The first time the knife on my leg required me to use the shears on my waist strap that was the end of taking a dive knife.
BC with integrated inflator/octo thingy. Abandoned that after a couple of dives.
Wrist slate.
New 80cuft alloy tanks that I took back to Oz when I moved home from the UK. No burst disc, no test pass and I couldn't find replacement valves. Blessing in disguise that one as I then changed to steel.
Sealife ReefMaster film camera and strobe. Never got to terms with it. Love my Fuji E900 + Ikelite housing.
Sunnto Mosquito dive computers. One for my wife and one for me. Identical failure (refused to enter dive mode) after about 18 months.
I much prefer my BP/W but don't regret my jacket BC that I bought during my OW course. It was the cheapest, lightest, simplest they had in the store and it's still in good working order ten years later. Perfect for travel. Still using the wetsuit, fins and regs I bought then, too.