Microsoft has disabled the option to change the default search engine on new Lumia Windows Phones. Previously, consumers were able to switch between Bing and Google depending on personal preference, which would alter which service the Windows Phone would utilize when searching the internet using the Internet Explorer address bar. After acquiring Nokia's handset division, Microsoft has made it so only Bing is available for selection.
We've checked existing Lumia Windows Phones that have been bumped to Windows Phone 8.1 and we're still able to select Google as the default search engine. Comparing this to the Lumia 630 (or other new Lumia devices), which has the search engine selection removed from the advanced settings area of Internet Explorer. If you're rocking an older Lumia and want to change to Google, open up IE, head into settings and then "advanced settings".
Update: It has been confirmed that some new European, carrier-locked Lumia 930/630 handsets still retain the setting.
Source: The Verge