My daughter bought a new iPhone and gave my wife her old iPhone 7 plus to replace the iPhone 7 my wife is now using with xfiniity mobile. Should I switch the SIM card to the new iPhone and get you to update the IMEI number or should I go to the xfinity store and get a mew SIM card for the new iPhone 7 plus?
It can't be done in the store. Call 888-936-4968. I went through the process recently, after I bought a new phone and gave my husband my old phone after his was damaged. The store was able to activate my number onto the new phone, but I had to call to have my husband's number activated onto my old phone. Make sure to call when you have some spare time, because it might take a while. I don't believe that's normally the case, but I think I may have had a unique issue.
See https://www.xfinity.com/mobile/support/article/bring-your-own-phone-details , about halfway down the page, Bringing a device to an existing line.