Hi @DavidTra-1199,
To answer your first question, yes you would lose those VNet integration capabilities. However, your VNET and subnet resources will not be removed.
Addressing your second question, when you scale back up, you will need reconfigure the VNet integration with your function app. They don't revert to how it was previously configured. Meaning, if you had your function app configured to an app subnet, you will need set that configuration back up.
To better understand why this is the case, when changing plans, your resources can get provisioned to different scale units. So, networking routing, more often than not, gets changed. If you want to have a repeatable provisioned environment, you can always use the Export template blade on the resource to create an ARM template.