Changing the application pool for a site running MOSS or WSS 3.0 should be accomplished through the Central Administration page.
1. Open SharePoint Central Administration
a. From Windows, Click Start
b. Click Control Panel
c. Click Administrative Tools
d. Click SharePoint Central Administration
2. In SharePoint Central Administration click on the Operations tab
3. Under the Security Configuration section click on Service Accounts
4. In the Credential Management section click the radio button for Web application pool
5. Select Windows SharePoint Services Web Application from the Web service drop down
6. Select the application pool you wish to modify from the Application pool drop down
7. Select the Configurable radio button and enter the new credentials