I am a h/j rider that likes a very flat seat. I have a couple of deep seat dressage saddles, and what’s important is that you find one that allows your pelvis to be neutral. Some will interfere in the pommel. Some will have too much angle in the seat up to the cantle. A more open seat design where you can sit with a neutral pelvis has a lot to do with the flap and the stirrup bars as well. For me, I have a long back and some dressage saddles make me arch my back too much, hurting my lower back, putting pressure up front, and then my long femurs get crammed into blocks on flaps that are too straight. A short block, forward flap, and a seat with enough of a flat spot in it where my whole seat can fit allows me to have a neutral pelvis and feels as good as any flat seat jump saddle. I think the longer stirrup takes more adapting than the seat, but there’s less room for error in a dressage saddle such that the wrong shape for your body is harder to compensate for.
I have seen them more commonly with the air. Wool flocking is an option when ordering new.
i’m a bareback rider and have a history of yoga and ballet. I don’t want a saddle to force my shape and hold it into a bucket seat. I want to move and flex and bend and train my horses. For that i need a flat, smoothe saddle.
Along with what has been said about getting used to a deeper seat …
Stirrup length, and learning to ride with a longer leg and more open knee, will matter a great deal to how any dressage seat will feel. If your knee is bent enough that it is bumping the knee blocks, that will push your seat back in the saddle and away from the center. That will be awkward in a deeper seat (of any style saddle). This is a very individual situation, as some people have bodies that won’t ever feel comfortable with a very long leg.
If you have a longer thigh bone (I do) then the blocks in front of the knees can matter a great deal. Same as above, the blocks push your seat back in the saddle.
It is definitely a transition to sit low and deep with a long leg. Frankly, suggest thinking about your overall riding ambitions. For myself, I’m not a pure dressage rider, and I’m not going to try to smash myself into a deep seat with big blocks. I’m not asking my horse to go in a more advanced dressage frame. So, flat seat, minimal block rider here! 
But if you want to go on in dressage (no matter what other types of riding you do with your horse), you might want to make that transition more effectively.
Interesting thread, especially since it’s been 35 years since I left the h/j world and joined the ranks of ‘just dressage’. I still train my horses to jump for the benefits of getting them to stretch their back and strengthen their hind ends; but, to that point, I still and always will prefer a flatter seat saddle. I have a difficult to fit breed (actually more than one breed); but, in my tack room right now, at least of the dressage saddles, sits a Roosli pilatus, a Prestige D1 and a Passier GG. I appreciate each of these saddles. Until they didn’t fit any of my horses I also had a Albion SL and an SLK. I quite frankly will never be happy with big blocks or a deep seat that tries to ‘lock me into position’.
@IPEsq and @OverandOnward: thanks for your observations and personal experiences. No ambitions about moving up in dressage. I’m starting to give up on jumping and want to focus more on having really nice flatwork. I’d be super satisfied if my horse would agree to get in the trailer (right, another topic) and be able to participate in some dressage shows (I quit showing in h/j and won’t go back - figuring out when your round can happen has gotten to be ridiculous). Thanks to everyone for the great inputs! I’ve been on the COTH forums for decades and love the community support.
As a “reformed” H/J rider who is learning to sit, I agree that a deep seat dressage saddle doesn’t have to be a problem, per se.
But!
They also tend to move the rider back a bit. If you look at the deepest part of the seat, it’s both shorter than a flat seat saddle and it’s farther back from the pommel and the withers. I disagree with the biomechanics of having a rider sit so far behind the horse’s center of gravity and behind the strongest part of his back.
I have some old Stubbens whose balance I love. But I wish they were softer and the fit for modern, wide horses was better.
I think you can find some french, Italian and Tad Coffin’s saddles that will let you ride right up close to the withers. And I think it’s easier to sit there as well as easier for the horse to carry us there.
If I had access to a lot of the British show hunter saddles, I’d try out some of those.
yes, this! I grew up riding saddleseat on Morgans and Saddlebreds and the feel and look of it always had a wrongness to it for me.
