If you try to change the identity for the applicaiton pool that the sharepoint site is running under, You can do this from the iis manager open identity tab and change the account for the application pool.
After you change this, you will get "Service Unavailable" error message when you try to open your sharepoint site, to fix this:
1) Make sure that you type the password for the service account which the application pool will run under correctly.
2) Make sure that the Service account is part of the following groups: IIS_WPG and STS_WPG .
3) Reset your IIS.......MOST IMPORTANT STEP.
then, you can navigate to your website.
Hope this tip helps.