Wow! I didn't even consider increasing the difficulty of the game after killing the boss! Originally expanding to new worlds and levels was our idea of increasing the difficulty, but for the jam, I really like your idea.
Thanks, yet I really hoped your answer would be that it does not because then my play through the game was totally hilarious. I killed the boss twice without ever dying, wondering what the store what for if you got back to the store only after completing the game, then only after dying once to the boss I realised (ok, I suspected it but anyway) that you keep your money so the shop made sense, but then I got killed twice more by the boss and started to wonder if it got stronger but with a deep conviction that it was not the boss which got stronger but myself which got weaker which is exactly what happened.