Local & Domain Accounts On Client Devices

Hello everyone,
I have a few questions about user accounts,
When a new device is connected to the WSE2012 server, the process states that the current local account is being migrating to a domain account.
What exactly happens during this process?

What if, as the network admin, I already have a user named John, and John today just got a new computer.
So all I want to do is remove John's existing computer from the connected devices, and connect his new computer. In doing so, when connecting a device, you are asked to provide user credentials. In this case, credentials of an existing user (John) are provided.
During the process, it still states the local account is being migrated. So what exactly is being migrated?

Moving on, once the device is connected, the user can now login with the domain account.

However, on the local machine, we can still type something like .\user_account (where user_account is the original local account) and log in on the machine using the local account.

So essentially, on the newly connected device, we can log in either using a domain user account, or an existing local account.

Is this the expected behaviour?

After the device was connected, I logged in using the domain account, and deleted the existing local account. Is this what you guys all do?

Thanks for the help!