means,

Be completely changed; to change from on form to another... in this case from a worldly person in enmity to God, to a godly person with a will conformed to the will of God.

I beseech you therefore, brethren, by the mercies of God, that ye present your bodies a living sacrifice, holy, acceptable unto God, which is your reasonable service. And be not conformed to this world: but be ye transformed by the renewing of your mind, that ye may prove what is that good, and acceptable, and perfect, will of God.
Romans 12:1-2

Being "born again" is like the metamorphosis of a butterfly... you are completely changed and become "a new creature in Christ". (2 Cor 5:17) But once you become a new creature in Christ, you also then must be not conformed to this world, but be "transformed by the renewing of your mind", as the verse above tells us.