You can do a report(advanced search) on that attribute for all possible values, It should give you the documents where its being used, AFTER that I would suggest you to remove that attribute from Type & Attribute Manager CAD Document object if your system has already gone live.
Understand that you just need to remove it not delete it from T&A Manager.
The bad part- I dont think you can delete that attribute even if you break the link in between. We had our moments dealing with this pain in the back thing..We just moved them to a Do not use folder and let them live there for eternity.
For breaking the link normally you may have to check in and check out the document object , by doing that you will see the old attr gone and new one will show up. Only when "removed" from the object and added with your correct attr. You can either automate the process or if there are less no of docs , manually do it.