I have a general idea with some key points from my local trainer, but I would like to see what others have to say;
What changes will the average horse have once they are started on Dressage and advance? I started going to a Grand Prix stable and all their horses' necks look extremely muscular, it appears that within the first two months of beginning Dressage for a horse that is already fit for pleasure riding at all gaits, the top of the neck grows and the topline starts to look like a straight line instead of a steep slant into the horse's withers. Another barn down the street says that horses lose their working trot for a while once they learn to lift their feet higher or collect a few strides.
Aside from the appearance changes, it looks like the horses can relax easier, and their front hooves have less cracks or chips. Personally, I've found my horse is reaching out far with her legs and is more alert when I give her verbal commands in the arena free lunging; since she figured out she could flex herself laterally or vertically but still move actively, she likes to stretch and bend far at will when I let her have a moment to get any gas out before a ride