I have already replaced my island countertop because the sink wasn't attached with enough screws and it separated. Of course water got into the gap before we could even see what was happening. Jayco replaced the countertop under warranty, but I had it shipped directly to me so I could install it myself. I didn't (and still don't) trust any dealer service to do it right.
To answer your question; the countertop cleats are stapled into the countertop and the sides of the cabinet. I was able to pry the cleats (3/4" X 3/4") carefully with a pry bar and H.D. putty knife. It doesn't matter if you damage the countertop you will be replacing, so you can be a little rough on that. I sealed the underside of the replacement countertop before installing the sink, and I did a very careful job of caulking the sink as I installed it (while the replacement countertop was on horses in my shop.)
One option with a replacement countertop, if you order one or make your own is to install the sink as a drop-in rather than undermount. It's easier to seal that way. I did mine as drop-in, which is better looking, but if this happens again I will take out the sink and install it as a drop-in.
If I had to replace it out of warranty, I would use 1/2" solid surface (Corian, etc) which would be less weight than butcher block. Of course, I have an advantage, having been a certified Corian installer for years; I have the tools and some leftover material.
