*Edit...This applies to United States orders for in store pickup...
If you ordered and selected Friday 9/16 and a pick up window between 12:00 and 12:30 you are 100% guaranteed that device is yours. Currently you should have already received a Confirmation email and it will be showing as PROCESSING. Tomorrow depending where you are, (but Thursday 9/15) at some point your status will and your Card will be Charged the amount you owe and your status will change. Phones are shipped to the store Thursday and even though you pre-ordered technically Apple cannot charge you until your item has shipped to the store. You should see the charge Thursday, however if you would like confirmation from Apple without waiting hours to speak with someone, do the following...
Call 1800MYAPPLE from the phone number you are putting the new phone on
-It will prompt you what you are calling for: Say Customer Service
-It will say, "We notice you recently placed an order for a new iPhone. Are you calling to get the status of your order?"
-Answer yes, and it will ask if you would like to hear your current status from read to you or if you'd like to wait for a consultant
-Just agree to have it given to you
-It should say that iPhone 7 is currently being Processed and will be available for pickup a ....Apple Store between 12:00 and 12:30.
*I'm in the exact same position but need to work so my window is 4:00-4:30pm.
*Our cards are pre-authed so they know the funds are available but they won't be actually charged till shipped tomorrow
*Rest assured that as long as you have ID and the CC used for payment, and apparently the owner of that card it will pretty smooth sailing.
*As for going early...it's a gamble. The stores are not uniform. In some instances there are separate employees doing pre-orders from those who line up early for "in stock availability". In other instances, you get there for your window and literally you get your device during that window because Apple has figured out on average it takes just under 30mins per customer between activation, trade ins of old devices for store credit potentially, buying a case, etc.
*In my opinion, the later in the day the better. You are guaranteed the device and the morning rush is done. I'll be able to sit down, plug into iTunes and restore from my morning 6s backup (I know...it's a new phone, do a fresh install...I understand the risks and appreciate the warnings but I'm certain my backup will be fine and there will be know issues.) Then I'll pair my watch and go on my way.