People's IPs change all the time. Some old accounts have hundreds of different IPs logged. It's not a problem, and you don't need to notify us if you move or change ISPs.
The only issue with IPs is if multiple accounts exist at one given IP and those accounts are completing offers. This is why we discourage the use of public or shared computers, because in many cases you never know who has used the computer/connection in the past.
As always, we evaluate your account and activity as a whole. We can easily tell when someone is using FC in good faith, or bad faith, and that's generally the determining factor for cashout rejection. In short - follow the spirit of the rules and be respectful to our advertisers, and you will not have any troubles.