I just ran into an issue yesterday where,
we have upgraded the environment from Portal 2003 to MOSS 2007 with Gradual upgrade everything is running just fine, but when we try to go into a sub site which has some lists inside it I try to browse through the lists and i find that the lists contain and interface of portal 2003
Just a little background here, the portal 2003 SharePoint site was heavily customized with banner and bread crumbs and quick launch bar, we took out all the customizations before we run the prescan which was successful. then we run the gradual upgrade and this bought the default pages of the site
The issue is I am running into is that the some document libraries still show the SPS2003 Interface. Initially you might think that this is a result of incorrect upgrade, you can try running the command
stsadm –o –upgrade –inplace –url <url of the site> –forceupgrade
If this does not help try resetting site to site definition from the top level site this works in 90% scenarios
I was able to get over the issue with creating a new view on my document libraries and which helped me get rid of the portal 2003 interface
That means when the upgrade was performed the Views did not get updated and sustained the old SPS 2003 look and feel so in short the default views got corrupt and so we have to work around it by creating a new view and replacing the default view.