Welcome, sj2016, and Happy Holidays to you! I'm glad you stopped by to ask your question -- the answer's a little complicated, so bear with me!
To participate in the 2017 Q4 Magenta Samsung BOGO Offer, you need to be a postpaid or T-Mobile for Business customer on a T-Mobile ONE or Simple Choice plan with Unlimited high speed data. This kind of data plan is part of T-Mobile ONE, but for a grandfathered Simple Choice plan like yours, that type of data plan would be an additional feature. I double checked the list of plans included, and while there are business plans there; the one that you're currently on isn't among them. The Simple Choice for Business 4 Lines for $100/$140 plan that you have would have come with either 2.5 GB or 4.5 GB of data per line, and I don't think there's an unlimited high speed option, so on this particular plan, you wouldn't qualify for the BOGO since you'd need to have unlimited high speed data on both the existing and the new line to be eligible.
I know you mentioned removing a line and adding one -- regrettably, our system won't recognize a change like this as satisfying the requirement to add a line. That said, if you're thinking of changing your plan in order to qualify for the BOGO, then you would no longer be limited to four lines, so this wouldn't be an issue -- you could add the fifth to qualify.
I know that the grandfathered plan you have is a great rate -- and if it's working for you, then the price shift to a newer rate plan might not make this promo the best option. I'm sorry that we gave you incorrect information over the phone; but I'm really glad that you reached out here to double check. Please let me know if you have any other questions -- while we can't access your account here to double check specifics, the info that you gave was enough for me to be able to look up the code and check it against our list. We're super happy to provide any other general info as well!