How Can Existing Home Internet Users Avail the Latest $300 Promotion?

This is the current promotion, and I want to take advantage of it. I am an existing customer of home internet, which is paid monthly without a contract. Can I just order a new line get this offer for me and cancel my old plan? Or do I have to cancel my existing plan and then reactivate it again to get the offer? I am confused about the second line of the terms below. 

Can any of you please help me understand this? Thank you so much and Merry Christmas!