bgp notification...

to get a notification message, there needs to be an error... changing the neighbor remote-as will effect such an error...

look at the message types in rfc4271:

Type:

         This 1-octet unsigned integer indicates the type code of the
         message.  This document defines the following type codes:

                              1 - OPEN
                              2 - UPDATE
                              3 - NOTIFICATION
                              4 - KEEPALIVE

 open
   After a TCP connection is established, the first message sent by each
   side is an OPEN message.  If the OPEN message is acceptable, a
   KEEPALIVE message confirming the OPEN is sent back.

 UPDATE messages are used to transfer routing information between BGP
   peers. 

 An UPDATE message MAY simultaneously
   advertise a feasible route and withdraw multiple unfeasible routes
   from service.

 A NOTIFICATION message is sent when an error condition is detected.
   The BGP connection is closed immediately after it is sent.

Keepalive 

 BGP does not use any TCP-based, keep-alive mechanism to determine if
   peers are reachable.  Instead, KEEPALIVE messages are exchanged
   between peers often enough not to cause the Hold Timer to expire.