Here is what we are doing.
1) Preordering from t-mobile website but NOT porting AT&T Numbers. The T-Mobile rep on the phone said we can port at a later time.
2) Once phones are received, we are piloting the t-mobile service. We did it once before, along with my brother and tons of friends having t-mobile.
3) Once satisfied, we are porting numbers.
Now here is the trick with switching to t-mobile, getting all the good deals, and keeping your cell service working until you get your new I6+:
1) You can go to the t-mobile store tomorrow and preorder. Pretty simple explanation.
2) Pre-Order online. However, if you port your number, your service deactivates right away. You can port at a later time. However, the rep on the phone said that once you get your new iPhone 6's, you can take it down to the store and return and resign up for the phones to get the ETF paid and Device trade in credit. He is unsure if you can get the ETF Credit and device trade in. Also you can do a Deferred Device Trade in and send in a device at a later date.
Note: I have not confirmed this, but the rep said that you can trade in ANY working phone with an IMEI number and get your ETF fee paid. He instructed me to go to Walmart and get a cheap pay per use phone, trade that in, and keep my iPhone 5. Crazy.
We will be doing #2.
We will be saving so much money on our phones. We talk a lot on our phones, but at home. We use our phones more as media devices. Games, Web surfing, Reading, Texting, Skype, Calendar, Working out, etc. T-Mobile is great for that.