Retire internal Windows root CA
Solution 1:
You need to decommission old CA from Active Directory using the following TechNet Wiki article: How to Decommission a Windows Enterprise Certification Authority and How to Remove All Related Objects
since your old CA is off, you need to complete only steps: 6, 7 and 9. Other steps are not applicable. After cleaning Active Directory from old CA remnants, make sure it will never be powered on.