I did this recently. My site was hosted in the UK, and webmaster tools was set to target UK. I changed the nameservers (from a UK nameserver and IP, to a canadian nameserver and IP); and also changed the web address IP to canada. At the same time I changed webmaster tools to USA rather than UK. Immediate effect after a few days was that my site dropped from page 1 to page 3. It's been 5 weeks or so now and the site is back at page 1 and steadily climbing in the US market, higher than it was in the UK. Both sites and servers were available during the cut over period. I changed multiple things at once which may have effected it: 1. Changed nameserver names 2. Changed IP address of web server 3. Changed webmaster tools preference for geo targeting