Try this. In ASPC, select Option 2 under Config File, and Get.
Now there is a line which says:
SipSrvID=x (whatever 'x' is...)
Delete 'x' from the end of that line, and click SET. This will stop the SIP server and you should be able to edit or delete your ASSDN parameters. If you have dual processors, do this for both.
Once you re-create ASSDN, you have to go back to ASPC, and put the 'x' back in and click SET to start the SIP server(s) again.