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I've been questioning this myself for my newest site, and opted to go with a stand alone IP just to be safe. Still, all the problems I've read about with virtual hosting seem to be a little outdated......
I have seen no evidence that page ranking has anything to do with having a dedicated IP versus sharing an IP*. The advantage to having a static IP would be that if another site hosted on the same IP address was spammy and Google blocked indexing content from that IP, then your site would not be indexed. This is, however, speculation, because I don't know for sure that Google bans indexing of IP addresses.
As for changing your IP address, or your server, or anything else behind the web address.. this should be transparent to Google because they access your website by name, not IP (this is handled by the domain name system). So as long as your website is accessible by name, then there shouldn't be an issue.
* - And I'm saying "sharing an IP" instead of "virtual hosting", because you can have a dedicated IP address using virtual hosting. The alternative to virtual hosting (which runs about $25/month) is to use a dedicated server (which runs about $200/month).
Craig Silverstein of Google says:Google handles virtually hosted domains and their links just the same as domains on unique ip addresses. If your ISP does virtual hosting correctly, you'll never see a difference between the two cases. We do see a small percentage of ISPs every month that misconfigure their virtual hosting, which might account for this persistent misperception--thanks for giving me the chance to dispel a myth!