To be fair, perhaps most sonatas include transposition within them, only it's written out. This actually presents a hurdle for me, because if I learned a theme

To be fair, perhaps most sonatas include transposition within them, only it's written out. This actually presents a hurdle for me, because if I learned a theme in G, and now I have to play it in D I feel like I know it, so I don't practice it as much as I did initially and it ends up in this weird limbo where I know what I'm supposed to play, but haven't properly trained my fingers. But there's plenty of stories of Liszt or Chopin transposing music on the spot. Jazz too.