Site Definition Woes

Can someone help me please, I have the following problem.

I developed a custom site definition solution in Visual Studio 2005 using the following process detailed in my post earlier.

I had no problems in my development environment and added the solution into our UAT environment using STSADM and deployed through Central Administration.

The template was successfully deployed and able to provision sites based on that template.

After a cycle of UAT some changes and bugs came back which I fixed in my solution and tested in the development environment no problems.

I took the wsp files and used the UPGRADESOLUTION operation in STSADM. I tested the provisioning of a new site and it worked, but when I checked an existing site that had been provisioned from the site definition the homepage just displayed a 404 - file not found.

I am still able to view the settings of the site if I access the _layouts pages. I have tried resetting the site to the site definition in the Site Settings with no luck.

I have opened the site in SharePoint Designer and can see all the files including the default.aspx site that cannot be found, but if I try to open it comes back with a soap error saying the file does not exist.

Nothing is entered in the event logs or SharePoint logs.

Please help I am desperate, we are meant to be going live in 2 weeks and am not very confident if I am unable to upgrade an existing site definitions through a solution. :-(