It might be very difficult to change.
Not because it is difficult in general - but if you are riding on snow/ice now, you might not be powered as much.
Thus your harness will have to take a lot more load on the water, and then EVERY little detail on the harness is critical - regarding how it fits you.
IF you ride extremely powered on snow, it might be the same for you snow and water, but most rides with a lot less load on snow and ice.
As others have said, personal what different riders prefer, ups and downs of both.
Seat is much more comfortable for most riders, and it does not ride up, and you can depower the kite more (at least you got more available travel if your bar allows this).
Some will also say you can jump higher, as you can hold more edge with the tow point lower (I second that myself).
You got a better view as you can twist your upper body more - really nice for everything.
So many, apart from those who because of their back have to, use seat harnesses. And when talking racing and hydrofoil power, always a seat.
Waist harness is the only choice if riding unhooked, as you can hook in and out without having to pull the loop down to an awkward low point.
You get more freedom moving your legs around, with a waist, thus for waveriding it is better, and you can save bad landings after jumps without crashing, easier.
So for freestyle and waves, waist harness is preferred by most.
Downside is it can slide up, a real PITA if this is the case, but depends on your body and harness and riding style.
Changing from one to the other, riding with the same power, is quite easy though.
Many of us use seat instead of waist for a long time, if we get a bent rib, till it is healed, or we change from waist to seat when going from waveriding to powerjumps or racing, or whenever we feel it is better one or another.
No problem at all.
The major issue is finding one that fits and works for YOU, and not another rider - more individual than anything else, so dont take recommendations for anything, you have to ride it yourself for a while, to know.
Our bodies are more different than fingerprints in this respect
Above is some of the differences I am aware about, hope it helps