Unfortunately, when you purchase and download music from the sprint store, they are licensed only to that phone and will only play on that phone. You can download the AAC+ files to your computer but they will not play on it, and once you have downloaded them once, you must repurchase them to download them again.
Now, I would argue that you don't want to download them to a different phone: you are still using the same phone, just with a different spring number. However, sprint musicstore accounts do go by your ten digit phone number, so technically a new number is a new account, thus a new phone. I recommend you demand satisfaction from a sprint musicstore customer service manager to see about account resolution.
One other point is that when you purchase music from sprint musicstore, you are entitled to a WMA version of the song you have purchased. This WMA file will play on your computer, and you can easily convert this WMA to an MP3 file, which will then play on your phone without digital rights management headaches.
I hope that helps.
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