I don't think anyone can answer and guarantee that answer for you. But if it was me? I'd go by my SC and talk with them. Explain yours is not under warranty and you are going to do it yourself. So, you want to swap SIMs to enable the LTE. They may expect (only do it this way) that you bring the old one in to get the new one enabled. I have heard that the SC can enable the new one - there. So, you might have to tear down, take the old one in, bring the new one home and install it to complete the process. Just a logical guess.