FWIW, I bought a slightly used 535 kit directly from ESS just before they announced the new 550 kit. After speaking with Roman, I arranged to swap the 535 pulley and belt for the ones on the 550 kit. I compared the pulleys and they are definitely not the same size. Also, the 550 runs more boost which isn't possible with just software. Regardless, swapping the pulley and belt isn't that expensive so this kit could be upgraded easily if the new owner wished, assuming it doesn't already have the smaller pulley.
Good luck with the sale!