Q: Really confused by iPhone upgrade Program

Hey dear friends, recently I am really confused by the iPhone upgrade program, please help me with my concerns,

Currently I have an unlocked iphone6s (i got it through ATT Next, and i paid off the phone, and they unlocked the device) and with ATT Monthly wireless plan. I want to switch my carrier from ATT to Verizon because I want to pre-order an iPhone7 and only Verizon iphone7 supports both GSM and CDMA.Then, i decided to enroll the iPhone upgrade Program so that i can get an unlocked iphone7, and also i can get an new iPhone every year (I traveling between different countries, so that an unlocked iPhone7 will make my life much easier) .

In order to pre-order an iphone7 I have to have a Verizon account so that I can finish the pre-order process such as wireless number, you billing zip code and SSN (last four digits). But I want to make sure I will get an unlocked iphone7 before switching to Verizon.

here are my questions:

1. Once I switch to Verizon and pre-order the iphone7 by using Verizon account and active the new phone with Verizon wireless service, will the Iphone7 remain unlocked? or verizon actually will lock the iphone7 again?

2. which side will actually charge the monthly fee of the iPhone upgrade program fee, verizon or Apple?

3. will it be easy witching carrier from verizon to other carriers such as, ATT, T-mobile or others?

4. once I switched to the other carrier, how should I keep paying the monthly fee for the program (if the monthly fee  for the program will be attached with Verizon Monthly wireless bill)

thank you for helping me!