[0des]

nope, this is a tried and true method i use to change my ip on non static service. I change the router mac address, flip the modem, and voila new IP

[sigmoid10]

Not necessarily. The fewer IPs in your ISP's dynamic pool, the bigger the chance that you will get the same one. In the extreme case when there's just one free address (I have seen this happen with some really shitty ISP) you'll 100% get the same one back.