Hi,
You could update all the sites in the database to use CKeditor
UPDATE mp_Sites SET EditorProvider = 'CKEditorProvider'
Then touch Web.config to clear the site settings cache.
Note that is is also possible for you to re-enable FCKeditor, but it is not being maintained by the developers and it has problems in newer browsers like IE9. We no longer ship the FCKeditor files but they are still on disk since you upgraded from an old version. It is commented out in the mojoEditor.config file in the root. You could copy that file and rename it from mojoEditor.config to myEditor.config and uncomment FCKeditor and then point to your custom file by ading this in user.config
<add key="mojoEditorConfigFileName" value="myEditor.config"/>
But my advice would be too use CKeditor since FCKeditor is not maintained.
Hope that helps,
Joe