I just have to say that I keep being pleasantly surprised by Ikea customer service! It does help to kind of have an idea of what they might actually do, and to be a bit pushy until you get someone who can help.
We realized this weekend that both of our lazy susans were seriously bent in the middle. They looked like someone had them on a pallet and was overenthusiastic about tightening the straps. That's $70+ of useless aluminum, there! We didn't notice it when we bought the 1st half of the kitchen in June, since the bent part was heavily covered in the shrink-wrap packaging and the rest looked fine. It's a subtle bend, but bad enough that they won't actually work. Not aesthetic, but structural.
So, I called Ikea up without much hope that I could get them to mail me replacements. We live 4 hours away, so the gas would cost more than the susans if we tried to return them in person. Ikea sent us a replacement drawer front by UPS for this same order, but that was a really little item! Anyway, the first couple of people I spoke with told me that I couldn't get a remote return at all since I didn't get home delivery. I knew that wasn't true because of the drawer front, so I kept pushing. I finally got someone who looked up the susan parts to see if they were small enough. They were! Half an hour later, I have an order number and will hopefully get two new susan shelves in a week.
We've been making progress on one of our key modifications: the ductwork section. Our stove will be on an interior wall with no venting possibility above, so we need to run the ductwork over 42" of cabinet to the exterior wall. One option is to cut a big hole in the 39" tall cabinet, but I'm uncomfortable with the loss of structural integrity. We decided to try to maintain the use of 39" high doors, but use a 30" cabinet so that the ductwork could run above the cabs. I'm very happy with the result!
Initial sketch of plan: 30" cab with 9" high extensions cut from a spare cabinet:

Cutting down a cab like this for the new sides (we used the cheapest wall cab) gave us the proper hinge spacing and holes - much better than trying to drill them ourselves!
Actual implementation - a bit beefier!
Side:

Front:

Top:

We'll put a top piece on it to pretty it up. It's actually MORE convenient than having a full 39" high cabinet going to the ceiling. Things can't fall into the back of the top shelf, out of my reach. We'll probably put some bulk spices up there.