I’m curious about this too. In theory, you could add a new line, transfer an existing line to the new one, then cancel the old one. In my case, I would move an existing iPad to the new line since I don’t care what number is associated with the iPad. I’m not sure if Verizon has some kind of waiting period with new lines where they don’t let you make changes for a certain amount of time to prevent things like this.
FYI, I posted my question about the BOGO, as a theoretical question since I know they would not comment on unannounced deals, on Verizon's forums and received the following response from an employee:
"We want to make sure our customers have the best deals available to them. With regard to your question in theory as long as the new line of service is active and not canceled you you would technically still qualify for a bogo promotion. If you were to decide to cancel another line and lose the number associated to it you would not be penalized for doing so. You cannot disconnect or transfer the service to an existing line and cancel the new service line."
https://community.verizonwireless.com/message/1606202#1606202