T Mobile Retail not honor

November, 2025. I see that is time for my son get a phone, so I went to T-Mobile retail store add a phone line for him. I had 4 phone line and a guy at store told me that they have a promotion if I add 1 phone line now then I will get a 2 phone line free and you can trade in to get a phone too. I told them that I have a promotion discount more than 10 year and wanted to make sure it will able to carried over to a new plan. the store's guy told me that will be no problem to carried over to new plan. I want to confirmed with him that I have a grandfather plan and the promotion been add 10 years ago. He said that no problem and he would take care that for me. later, I received a bill but my discount did not showing. I came back to store and told him. he told me it would take time to transfer over and waited for the next bill. I came back after I received 2nd bill with the same issue. he gave a store credit for those discount and he said he would working with T-Mobile to fix it (please did not contact T mobile customer service he would take care of that problem for me). he gave me store credit every month and told me that He still working with T-Mobile customer service on my problem,  after 5 month the problem still there so I called T-Mobile and found out because I have a grandfather plan and promotion discount would not exist anymore, so T-Mobile customer service wont able to add it back to me. I still do not understand that T-Mobile retail store (representative for T-Mobile) made a promise to me that he will able to transfer my promotion discount from my old plan to new plan. . Now T-Mobile retail store keep delay it sound like he wont able to do it and T-Mobile customer service told me that they wont able to do it. it made me upset and disappoint to T-Mobile about the communication between their department. if i know that i will lost me discount then Ill upgrade a new plan.