Ok gang, I just got back for the T-Mobile store. I would have typed this up in the parking lot, but I was logged out of MacRumors on my iPhone and couldn't remember my password.
Background: I'm a longtime AT&T customer with a grandfathered UDP. I have two 5s(s) that are on a 2-yr contract with AT&T until mid-December. I wanted to sign-up with T-Mobile today so I could pre-order a 6s and a 6s Plus tonight as a Jump On Demand customer with T-Mobile.
Here's the scoop:
At the T-Mobile Store, they gave me two temporary SIM cards with T-Mobile phone numbers so I could call in and pre-order tonight. I picked my plan, in this case the two lines with 10GB of LTE for $100 and that was all I had to do. I walked out with two Sim cards/phone numbers and I can now call in tonight. When I call in tonight, all I have to do is pre-order my phones and say that I want to be on the Jump On Demand program. I also have to let them know which phone I'm trading in if I want T-Mobile to later reimburse me for my ETFs with AT&T. You can trade-in any working phone that doesn't have a cracked screen and in my case, I'm going to trade in an old iPhone 4 and an iPhone 4s. That will satisfy the trade-in requirement and then I will sell both of my iPhone 5s(s) on the side. However, if you do want to trade-in your existing phone with T-Mobile, they are giving the best trade-in value available and will lock in that price with you tonight when you call. If you can find another buying service in the next 7 days that will offer you more, T-Mobile will give you the difference plus an additional $50.
While I'm waiting on my new iPhones to arrive, I can still continue to use my existing iPhone 5s(s) with AT&T. When my new iPhones arrive (whether it's on Sept 25th or 6 weeks later, all I have to do is drop the SIMs that I was issued tonight into the new phones and port my AT&T phone numbers over at that time. Also, when I receive my new iPhones, I will also receive a white plastic bag to ship my trade-ins back to T-Mobile. So, if you go this route, you will not be without a phone during this entire process.
The only possible hiccup could be when they start the bill for my service with T-Mobile. They shouldn't start it until I insert the SIMs into a phone, but if for some reason they do, then I'll have to give a call to the T-Mobile manager at the store and have him put in a request to credit my account. So, if they start charging me now, it doesn't really matter and I'll be covered...it will just take a phone request on my part to get things cleared up.
So there you have it. This process will work and for those of you that still have the time to get to a T-Mobile store tonight, you can do this too. If you can't get to a store or of they are already closed, have no fear. You can call T-Mobile tonight and do the same process over the phone. However, it would be in your best interest to do that in advance of the pre-orders starting and then call back to actually order the phones when 12:01 am PDT hits so you can complete the process sooner and secure your place in line.
I hope this helps -- good luck to those pre-ordering tonight!