I was in the same situation as you when it came time to upgrade to a more permanent pool. The old one was 48" tall and the new was 52". I didn't want the deck shorter than the pool so the only option was to dig out the ground and lower the pool. Luckily the slope of the land ended up still working in our favor and the low side was almost right at the perfect elevation, but we also went bigger so it went out farther.
Your situation may not work out the same if staying with the same diameter. If you know the finished height of the top rail then using a transom or laser level and digging to the right depth wouldn't be so bad to get it to line up how you have it now. It's just a little more challenging, but it can be done. Trust me!
Also wanted to point out that going from 18 to 24 only required slight modification of the deck. But ours doesn't go around as much as yours. And since like you said they don't do 16' putting an18' will probably result in a gap more in the middle of the deck between the pool. If you put the pool in where it is taller than the existing deck than that would definitely make it easier to frame a small step up all around and cover any unsightly gaps as well as visually blending the new pool into the old deck.