Nice take on the theme! The sound effects were great and, together with the exchange mechanic, it really reminded me of Mega Man. I like the boss variety, though as already said by others, the red, yellow, and blue bosses were too easy compared to the rest, which didn't really incentivize the player to use the shift mechanic. With some balancing and some polishing on the art, I could see this becoming a really cool retro game!