There is no guarantee that it will change, or won't change. That largely depends on the network. Changing the device connected to the modem, sometimes triggers a change, as does ordering a different speed service. As indicated above, oversubscription of a UBR might cause a change. Realistically IPs are assigned on a "lease" of a few hours or days (depending on the network configuration). If you disconnect long enough, your lease expires and if someone else connects in the meantime and grabs your old IP you get a new one when you reconnect.
http://www.cableforum.co.uk/board/94...p-address.html