Jishman, I get the concern over wanting to be safe and not losing your BOGO upgrade discount on the new line of service. To help clear things up, no you will not lose your monthly discount for swapping SIM cards or even the device on your account. What matters is that you keep your device payment agreement active for the full 24-months on the new line you added.
Question #1 and #3 at the link attached under Buy One, Get One Promotions (BOGO) helps explain the requirements to retain your discount. This will help reinforce your concerns and ensure no mistakes are made going forward. Does this help clarify your BOGO concern?http://spr.ly/6602EXE4Q
If my response answered your question please click the _Correct Answer_ button under my response. This ensures others can benefit from our conversation. Thanks in advance for your help with this.