AT&T to Verizon - iPhone Upgrade Program

We've had AT&T for a long time and just got our new iPhones through apple under the iPhone upgrade program (iPhone 11 pro and 11 pro max). The problem is since upgrading, our AT&T service has been absolute garbage. Phone calls aren't working well, internet is very spotty, worse than our previous iPhones.

We are thinking about switching to Verizon but I want to be sure its even technically possible with our phones and how others might have done it in similar situations.

I'm not sure if:

• these phones are GSM only (AT&T)
• AT&T will unlock them under 60 days after adding them
• they will work on Verizon CDMA network
• this will have any impact on my upgrade program