Thank you for your question. I look forward to working with you to provide you the information you are seeking.
When a customer upgrades a phone, it is up to the service provider as to whether or not their new phone would continue to be grandfathered under the old plan, because you are signing a new two year contract to get the phone at the discounted price. If you had to sign the new 2 year agreement to get the new phone, they do not have to grandfather you on the old plan because getting the new phone constitutes a new agreement. However, you at the very least have a right to call them and negotiate with them regarding this matter and you by law have 30 days to return the phone and go back to your old phone and old plan and you can do that as well if they refuse to negotiate a different price with you. However, legally you do not have a right to insist on being grandfathered on the old plan because of the new phone.
I truly aim to please you as a customer, but please keep in mind that I do not know what you already know or don't know, or with what you need help, unless you tell me. Please consider that I am answering the question or question that is posed in your posting based upon my reading of your post and sometimes misunderstandings can occur. If I did not answer the question you thought you were asking, please respond with the specific question you wanted answered.
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Also remember, sometimes the law does not support what we want it to support, but that is not the fault of the person answering the question, so please be courteous.