My iPhone 7 was bought from AT&T. I would like to change to Verizon

AT&T would have to unlock your phone if it is locked to AT&T.

Once that is done, Verizon should be able to provide you with a new SIM card.

Remove or switch the SIM card in your iPhone or iPad - Apple Support

The AT&T iPhone 7 is not compatible with Verizon's CDMA network. Even if you get it unlocked by AT&T, you will not be able to use it on Verizon.

The AT&T iPhone 7 lacks the modem to connect to Verizon’s network.

https://www.cnet.com/news/intel-modem-chip-in-apple-iphone-7-att-tmobile/