Migration - Service Applications

Hi All -

After the Central Admin site has been created. Do we need to create new service application in 2013 and then do the upgrade process (I mean the DB attach)? If we will create a new service application won't it create a new database here in 2013? Do we need to replace the database? I am sorry but confused here.

Please suggest. 

Why do we need to use power shell and not use Configuration Wizard instead for creating the service application?

Thanks,

Khushi

Khushi