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Ironic timing. My wife was offered the unlimited plan and it had the option to add me. We did that and added the g1 data plan to mine, and I was able to get the g1 itself for the promo price. I'm typing this on it now.
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You can change to whatever phone you like by just moving the SIM card to a TMobile locked phone or an unlocked GSM phone. However, if you are going to get a phone that has a data plan (like the G1), TMobile will force you to get a new voice plan. It's not really because they're mean. Their system simply will not allow them to make changes to an old plan without updating it to the current plan types.

So, I'm at a crossroads. My four year old Razr V3 is dying, and I'd really like to have PDA and smartphone features in whatever I replace it with. Right now I'm looking at the HTC Dream/T-Mobile G1, but because I have a SmartAccess 1500 plan that is no longer available as a contract, T-Mobile wants to charge $399 for the phone unless I change plans to something that costs more and has less minutes. I haven't even gotten to the data plan part.

I found a vendor that sells the phone, for the T-Mobile network only, for $299. That doesn't seem quite as bad. What I'm not sure about is how to go about getting T-Mobile to acknowledge that handset change (if necessary, I haven't looked into what just moving the SIM will do or if that's even possible) and if there's something I need to do special to get them to drop the existing text messaging plan and adding the G1-specific data plan.

Any info from anyone who's gone through this would be much appreciated.