Do I need to disable a user when they are moving within the same Cabinet (Ex: AAAA to AABA)? 

No. If a user is moving within the same Cabinet, you will only need to create a modified UDOC that will switch the Department field within Home Organization tab in the Header section of the UDOC.  Use the InfoAdvantage Reports SEC-008 and SEC-009, located in the Statewide Security and Workflow folder, to review the user's Foreign Org, Security Role, and Workflow Role assignments in case any need to be modified as a result of the department change.