T-Mobile Loyalty

There are a few qualifying things to keep in mind:

1. If you are on a legacy plan (a plan that is no longer available) you generally are not allowed to add another line under the same plan since it is no longer available. That excludes your eligibility from "add a line" promotions which are generally tied to BOGO deals.
2. You can always change your legacy plan to a current one and get the same add a line promotions but doing so may increase what you pay for the plan and/or lose plan benefits that are no longer supported.
3. As for phone upgrades on existing lines, I've always been able to get same promotional trade ins without the need to change plans. However, you need to have established credit on TMO that can cover the purchase price of your upgrades. If you want to know how much credit line you have, just ask a TMO support rep. The way this promotion is structured it highly incentives you to stay on TMO for 2 years. This is something TMO wants for new and existing customers.
4. For promotional "pay up to xxxx to transfer to TMobile", those are designed to pay off cancellation fees and equipment balances. Additionally, you only get your equipment paid off if you buy brand new TMO phones that will be adjusted to what you owed from your prior provider. Basically, you don't make money on transfer. There is nothing here that applies to existing customer since the benefit is zero dollar value.
5. If you believe that existing customers should get promotions that are not offered to new customers, that is already in place - TMO allows you to keep your legacy plan for as long as you want. Additionally, depending on your legacy plan it automatically adds new features and benefits on new plans without changing your plan and price. When I started on my current plan there was no international roaming, airplane internet, and Netflix. Today I have all of those and don't pay a penny more than day 1. On the flip side, features on your legacy plan are never taken away even if TMO stops offering it. I have a cash back feature on every line that gives me $10 back in credit if the line uses less 2 GB of data. This is not available to new customers.

At the end of the day, you decide who is worth your hard earned dollars so if none of the above has value to you, then you should consider switching providers. That will make TMO take action if more people are leaving versus coming. You can call TMO and use a "soft bluff" by telling them what you want or you will take your business elsewhere. You just have to be serious of doing just that if you don't get what you want. If you are not specific about what you want, you are likely not going to get it.