Specifically with the yahoo transport, for some reason the buddy list is only synchronised when you register with the transport. So the easiest way to get it to update is you manually re-register (it should have all your existing details, you just need to hit ok), and it should import the new contacts.
I have one more question in this regard. I need to authorize the buddies
that I already have added in my buddylist in that transport. I could make this transparent by enabling auto authorization. But there
is one catch. If I enable auto-authorize won't I be auto authorize spammers as well which has added me. Is there a way to distinguish between people who were already there in my buddylist in that transport and people who has just added me in the transport. For people who were already there in my buddylist I could auto-authorize them. But people who has added me newly, I want to authorize them manually.