As Chuck points out, you may or may not lose draw length switching to a longbow. Most recurve grips are thinner at the throat, say 1 5/8 to 1 3/4 inches deep, vs about 2 inches or a little more for a Hill, so you could actually add a quarter inch or so in that regard.
On the other hand, a Hill is also lower wrist than the medium grip you have now, and is likely to shorten your draw a bit more than 1/4 inch.
I'm a bit puzzled by your definition of low wrist style as (pressure on the heel of my thumb). Low wrist is actually pressure on the heel of your palm at least equal to the pressure on the web of your fore finger and thumb.
Regardless, I shoot both, but haven't really checked to see if my draw length changes.
I cut all my arrows (carbon and wood) 1 inch longer than my draw length and then tune with point weight and/or side plate thickness modifications. Since I've been at this a long time, I start with a spine that for me is very close to being correct, if not right on, to begin with.
Agree with Chuck and Deno as well regarding playing around with some of your old/junker arrows first to find what spine is going to work for you before you buy.