I have a "Pay As You Go Legacy account.

I don't think the "correct" answer to this question is correct. I have upgraded my phone to my wife's better phone making it a used phone upgrade) and it works fine. The upgraded phone has to be an authorized T-Mobile phone and you have to install your existing SIM card. That's what I did and it booted right up.

snn, your presumption is not supported by the poster's question. As in my case, I upgraded an older phone to a newer phone; the fact that the newer phone is not brand new does not negate the fact that it is an upgrade from my older phone (I also drive used cars..:-).

OP wanted to know if the plan would change. The plan wouldn't change unless a promo (as gramps28 stated) required a plan change to get a new or cpo phone at reduced cost or on EIP.

Yes you are also correct that a newer model can be an upgrade even if it is paid in full and clear.

However. The question was in regards to a plan change because the OP wanted to upgrade a phone.

A phone can be BYOP or paid in full on prepaid plans provided the are T-Mobile approved devices. However a phone on EIP would usually require a post paid plan.

michael3442 wrote:
snn, your presumption is not supported by the poster's question. As in my case, I upgraded an older phone to a newer phone; the fact that the newer phone is not brand new does not negate the fact that it is an upgrade from my older phone (I also drive used cars..:-).

Hi @michael3442

Welcome to the community forum

Thank you for taking your time to join our discussions. I  think there might be a little misunderstanding. I believe that you took issue with the statement @snn_555 made that  " you'll have to buy your phone at full price".

I believe that you thought that @snn_555 was saying that the OP would have to "buy" a phone in order to use it on his original plan. You responded that You were able to transfer your sim card to your wife's former phone without having to buy a phone at all, and it worked fine without being required to change your phone plan.

What @snn_555​ was in fact saying, If a person was buying a phone (not using one obtained for free), he would not be able to buy it on an installment plan. He would have to pay full price. If he wanted to buy it on an installment plan, he would have to switch from his existing phone plan, which did not allow purchases on credit.

Thanks for taking the time to join the forum and remind us how easy it is to switch phones without switching plans. That is certainly a benefit of using T-Mobile where you can switch phones by a simple sim card swap.

Art

You may need a new SIM for the new phone depending on it’s capabilities. However, that should not effect your account unless that account was strictly for the SGH-t219. Also, I’m not sure you’ll get internet (data) except for wifi on your old account.

That’s the best way to find out if it will work..TRY IT! 

Unless your SIM is quite old, you should be able to pop out the nano size SIM from the adapter it came in, try it, then put the adapter back on (kinda tricky), and put it back into your SGH-t219 VOILA!