I’ve never heard of any regs that you could just "add-on" a cold water kit to make your regs sealed. Have you used it yet? If not, just return it and get the SC version. As far as the working temps for that reg, I would say you could take it a little under 50° as long as you stayed shallow. I wouldn’t want to test it thats for sure because having your reg freeze up on you at depth is not fun at all. No matter what you decide to do, I would stay away from that shop you bought from. Sounds like they are more into pushing sales than helping the diver out.