If multiple QACs exist in a forest, QAS will only use the oldest one that can be found. If you just shut off the DCs in your retired domain where the QAC is located the old QAC may still be found in the Global Catalog preventing any new QACs created in the forest from being used.
Due to this, if you are retiring the domain in which your QAC exists make sure that you delete the old QAC before retiring the domain.
Before removing the old QAC make sure that you have a copy of your QAS license and that you record the "Unix Global Options" and the "Custom Unix Attributes" preferences in your Control Center so that you can set them up again in the new domain.
In order to do remove the QAC you can use the Remove-QasConfiguration PowerShell command which is installed on Windows systems which have the QAS Control Center installed. Once the old QAC is removed you can create and configure a new QAC in another domain.