A document type key field cannot be changed using Administrator if there are any Contentverse documents saved using that particular document type.
For document types that have no current documents in Contentverse, under the Document Types tab in Administrator , using the update button, you can then change the Field Type of the new Index to Key. The field type for the Index that was previously the Key field will change to required. You can change this to "Not Required" if necessary.
Note: If there is a major reason to change document type key fields after documents have been entered for a particular document type, Computhink Professional Services can work with you to change the key field via custom database scripts.