Converting from postpaid monthly billed to prepaid T-Mobile Prepaid
To convert from postpaid monthly billing to prepaid contact customer care (611 or 800-937-8997) and tell the representative that you wish to convert your account from postpaid billed to prepaid billing. They will contact the technical support department and convert your account. A new SIM is not necessary. Technical support will convert your account and you will also get a $5.00 credit.
As of Feb 2010, I did not get the $5.00 credit. However, that may be because I attempted to switch a couple days before the contract was up. Even if the representative assures you that there will be no problem, DO NOT let that happen! They charged the early termination fee, and although everyone I talked with was very understanding, it was 3-4 months before I got it refunded.
Also, although the service at the retail locations is always excellent, prepaid 611 support is handled in India vs USA for postpaid. The departments are completely separate with different phone trees. Be prepared to use 611 less. If you do have an issue with the postpaid account after switching (such as I did), don't even bother with 611, as the reps in India will not know how to transfer you to postpaid and you will go in circles.
Source(s): T-Mobile solutions