Great question, Ryan37. As long as the device payment agreements are still on the original lines and not altered in any way, you are able to switch out the devices on the lines. Does this help answer your question?
You will need to purchase the iPhone at full retail price.
I'm most definitely NOT a VZW employee. If a post answered your question, please mark it as the answer.
We want to make sure that we provide the most accurate information, Ryan37. Can you please provide more details as to what you are trying to do? Will you be purchasing an iPhone XR from a third party, or at full retail? Are you looking to simply activate the iPhone XR on one of the lines that has the Galaxy Note9 active?
I also do recommend confirming that the device is compatible with our network prior to purchase (if purchasing from a third party). You can do this here: http://spr.ly/660416NlI