There aren't many FantaSea owners here. Luckily, I am one of them, and I do have a deep end. You will probably need to make some changes, but it should be easily achievable. FantaSeas are sold by LeisureLiving.com based in Grand Island, NY, near Buffalo. They'll tell you probably the same thing.

You'll notice that the bottom of the structure is a heavy steel u-shaped beam that the uprights sit in. The water pressure on them is enormous, so across the bottom of the pool, under the sand, are 1" wide belly straps that keep the side walls from pushing out. There is almost certainly one or two where you want to put your deep end.

You cannot simply remove it.

Instead, there is a large bow-shaped truss out of heavy steel that gets attached to the outside of the U-channel where the deep end is. There's one on each side. It's about 18' long, and sticks out in the middle about 18" and triangulates the beam so you can remove the belly bands across the deep end. Then, it's just a matter of digging it out, and re-filling it with sand to spec, and installing the new deep-end liner.

They make them for most of the standard size pools, and even can make it for my custom-sized one.

They did STRONGLY suggest, when I built my pool, that I go with the 5 1/2 ' flat-bottom deep and not the conical 7 1/2' foot one. Not sure why but I suspect it's harder to keep the deeper one clean. We have found the 5 1/2 foot to be just fine.

Carl