I really don't know, but since the Surfliner is unreserved, I'm inclined to say yes! Unless that AGR ticket is for BC, then no. (BC is all reserved, and only good for that specific train.)
I just had the same question as the OP. I'm not sure what BC means, but I've got an OSD>LAX ticket that I didn't use on this past trip that I'm hoping to be flexible with on an upcoming one. Can you clarify? Thanks