The IP address of a website is the numerical representation of the location of the server which is hosting the website on the internet. Depending on the hosting environment, the IP address may refer to a single website on a single server (dedicated IP) or it may refer to multiple websites on a single server (shared hosting environment). Its up to the server to decide what website to display, based upon the domain name. The domain name is "pointed" at the server (using DNS), when you type in the domain name, it takes you to the proper server, then the server displays the website.
When you change an IP, a new IP will need to be assigned in order for the server to be internet accessible (in order for the server to serve websites) and the old IP is freed up. Normally, hosting companies do not disable IP's that are not in use, rather, they are just free IP's reserved for use by that server or hosting company for whatever the next need is.
