Lets say I have a Person Entity.

I want to make him an Empoyee Entity (Employee Inherits from Person).

He was originally only a Person now he is an empoyee and needs to have his ID into the Empoyee Table

How can I do this with LLBL Entitys that doesn't involve copying over the fields?

I'm afraid that's not possible. Once an entity is created with a specific type, you can't change that type later on. So you are left with the other option -> remove the Person Entity and add an Emplyee Entity