I'm trying to change the services and app pool user, and it seems to work just fine for all except one site.

The SharePoint site just shows the "Error: Access Denied" page, and I see this error in the Event Log:

Source: ASP.NET 2.0.50727.0 Event ID 1314 w3wp.exe
Category: Web Event
Event ID: 1314

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Event code: 4011
Event message: An unhandled access exception has occurred.
Event time: 9/7/2010 10:50:14 AM
Event time (UTC): 9/7/2010 2:50:14 PM
Event ID: 0994d9ce6b364976a2820ccaf7d8e712
Event sequence: 4
Event occurrence: 1
Event detail code: 0

Application information:
Application domain: /LM/W3SVC/151433534/Root-1-129283445476278713
Trust level: WSS_Minimal
Application Virtual Path: /
Application Path: C:\Inetpub\wwwroot\wss\VirtualDirectories\100\
Machine name: (sharepoint server name)

Process information:
Process ID: 1840
Process name: w3wp.exe
Account name: DOMAIN\(new service username)

Request information:
Request URL: http://(servername):100/Pages/Default.aspx
Request path: /Pages/Default.aspx
User host address: (ip address)
User: DOMAIN\(domain admin user)
Is authenticated: True
Authentication Type: NTLM
Thread account name: DOMAIN\(new service username)