I see your point. Ok let me take a stab at it. If your extra line is under same T-Mo ID or a different T-Mo ID, you can login to Multi-Line settings of S7 and can access it as DIGITS. You really do not need to convert that voice.data text line to DIGITS. If I remember correctly that promo was meant for voice line and the moment you convert that voice line to DIGITS only line like a DIGITS Talk and Text Line, you will loss that promo. So its either way will be lost as you are removing a voice line anyway. Does that make sense to you? If not please post your doubt and tag me,.
This is a great question! Typically the only requirement to maintain your promotional eligibility is to keep the line active. Since converting the line to a DIGITS Talk and Text line or Data with Paired DIGITS line is not really canceling the line, I don't see why you wouldn't be able to convert the line. I would say hold off on converting the line for now as DIGITS is fairly new and I want to get further clarification on this. I will follow up with you here as soon as I hear back from our support teams on this.