As someone who has done both (and is still doing both with my young horse!) it is a BIG change. There is a lot I like about showing dressage, #1-5 being RIDE TIMES. You know exactly when you’ll be going in the ring, so you can plan your day out according to that, no waiting and waiting and waiting. It makes it so much easier, especially if you’re just trailering in for a one day or schooling show–if you’re lucky you can be in and out in a couple hours instead of all day waiting around.
Your hunter show attire (even field boots!) is totally fine to start off. In a lot of ways, I think now dressage attire tends to be a little more open to rider preference versus the more uniform hunter look you see now (it did NOT used to be this way–when I was younger dressage was all black and white, while there were a lot more colors in hunters with patterened coats and matching shirts etc). You will probably want to invest in fullseats for schooling and showing though–IMO as someone who switches between jumping and dressage saddles regularly, while I absolutely can and have ridden in my dressage saddle in kneepatch breeches, that fullseat grip works as an extra reminder to my muscle memory/brain to keep my bottom in the saddle and not revert to half-seat.
Braiding is also so much faster and easier. I can do dressage button braids in less than half the time it takes to do hunter braids. I refuse to do the unicorn forelock thing though and do a regular hunter-braided forelock