I'm also on a more challenging horse recently: a younger, sharper mare who is "definitely not a novice ride" according to my instructor! I take it that means (according to her) that I'm not a novice anymore, which is flattering.. but it's a *little* hairy at times.. her canter is so, so much bigger than the old chap who was my regular ride it's exhilarating and scary, and much harder to sit to (to be fair I'm a bit big for her usual dressage saddle, so I've asked if there are other saddles on the yard that would fit both of us. Despite the fear... I think I'm in love! She's mare is so quick off my leg it's amazing. I have to remember not to use any once the trot is established.. when I dismount at the end of a ride I realise I have not had to work nearly so hard.
It's good to ride different horses, and you have to trust your instructor when they say you are ready to step up, but I know what you mean about it feeling like starting all over again!