I rode H/J's for 9 years before I started Dressage. I would just like to say, it will benefit you and your horse A LOT! My position has improved immaculately from Dressage, my horse is a lot more comfortable and I understand his body so much better, needless to say I love Dressage!
Just listen to your trainer, ask questions, make your mind a sponge!
And P.S. Sitting in a Dressage saddle for the first time, makes you feel like your sitting in a bucket if your used to your jumping saddle all the time lol!!