Changing application pool id in IIS is not at all recommended way of changing App pool identity in SharePoint 2010.
If you directly change App Pool Identity in IIS, SharePoint will not understand it and will be still referring to older ID.
To change the identity for an application pool, log into Central Administration and follow these steps:
1. Go to Security and under General Security, click Configure Service Accounts.
2. Select the application pool from the components drop-down
3. Select the managed account that you want to use as the identity for this application pool, or register a new managed account in SharePoint 2010, using the Register a new managed account link.
4. Click the OK button.
5. You’ll be warned that this action requires an IIS reset on all servers, click OK.
6. Perform iisreset /noforce on all WFE servers in the farm.