I would imagine that you will need to buy a MK3S now as I suspect they have no upgrade kits for the MK1 left. This technology is advancing so quickly that they can't provide the upgrades for long, and the newest models are to different to upgrade to from a MK1. I would give Customer Services a go using the online Chat facility on the shop homepage (https://shop.prusa3d.com/
), it's in the bottom right corner but can take a minute or so to appear. They may
have some left in stock but don't
quote me on that as I have no actual idea! It has been 4 years now since the MK1 1.75mm came out, and all together there have been several iterations since then. When they started they we a small company and probably couldn't afford to keep large amounts of reserve stock. I have the plain old MK2 (no S or .5) and am planning on getting a MK3S or MK4 depending on if/when a larger print volume version should be released.