No LTE/4G after porting # to new carrier

I need to port my Verizon iPhone 5 to a new carrier as the service & coverage is much better here on AT&T.

I am wondering if I can replace the current Verizon Cellular antennae with an AT&T cellular antennae. For instance, with this part:

iPhone 5 Lightning Connector and Headphone Jack

I understand that I will need a new AT&T specific SIM card however, this should be easy to obtain when I actually port my number to AT&T.

For a little bit of background related to my question above, I have heard rumors that swapping the cellular antenna in an iPhone 5 will reduce the speed of the data

connection over cellular to half speed or less. Is this true?

Thank you in advance, the experts on this site and the information (and parts made available) has saved my tail more than a few times. My hat's off to all contributing staff and members of iFixIt!!!

Thanks again,

_rs