Tim,Unfortunately there is no direct tool to change Domain Owner for a geodatabase.You can try below workflow:1. Export existing domains owned by 'X' to a table using 'Domain to Table' tool2. Remove the existing domains owned by 'X' from the fields in the data3. Delete the domains owned by 'X' connected as user 'X'4. Import the domains from the tables created in step 1 using 'Table to Domain' tool connected as the "DBO" user5. Assign newly created domains in step-4 to fields from which they were removed in step 2This works successfully for me. However do take a backup of the geodatabase before trying this at your end OR test it on a test geodatabase and then implement it.Domain To Table (Data Management):http://resources.arcgis.com/en/help/main/10.2/index.html#//001700000022000000Table To Domain (Data Management):http://resources.arcgis.com/en/help/main/10.2/index.html#//001700000025000000Hope this helps!