Hello Mohamed,
Unfortunately, if your data source is currently ABAP then as per note 718383, a change to ldap or UME only data source is not supported. In a dual stack environment, ABAP data source is mandatory.
It can be possible to change this however, it can lead to many issues and inconsistencies with User and roles etc.
Alternatively, you can still achieve SPNego authentication vai AD with an ABAP data source.
Please have a look at the following Blog explaining the procedure to setup SPNego with ABAP Datasource
http://scn.sap.com/people/holger.bruchelt/blog/2008/03/10/configuring-spnego-with-abap-datasource
That should get you started with configuring this.
Hope this helps.
Regards,
Dave