I have 4 active lines on my account, two of them are BYOD. I am planning to use another iPhone with one of BYOD line, my guess is that Xfinity will be able to activate another device on the line with no extra efforts.
I had chatted with chat support and they were not able to understand my simple request.
Read through the forum and you will see that there are a lot of simple requests. What happens after that can be anything but simple, especially with byod.