They do capable of changing your ip whenever they needed to.
formally, your ppp connection will be disconnected and re-connect again, modem will re-authentation the username and password, and then u will get a new ip.
BUT, sometimes, they just simply re-assign a new ip to u without disconnect you from their server.
it seem seamless change to them, but it cause more security issue. Some applications and websites will detect this change, and assume u r crewing with them or ur system is compromised, so they limit or block your access to their system until futher identity comfirmation(through email, sms, ic copy,human face ..,blah...blah..blah..)
But the truth is, TM is screwing with us.. the ip change without discconect happen again a few days ago.
i think it is very lucky to me, only pan.baidu.com was affected this time.
it only need to re-verify through email, not too hassle ...

i have to re-register my ip with getflix everyday... not an issue for me... just curious..