In your migrated pages, you should see them all work correctly. However, the moment you go to do something in the new Page Layouts of SharePoint 2010, you will quickly find that it doesn't work. The new register directive sets the tag prefix as SharePointWebControls, NOT as SharePoint.
In 2007:
<SharePoint:FieldValue FieldName="CompanyUrl" runat="server" />
In 2010:
<SharePointWebControls:FieldValue FieldName="CompanyUrl" runat="server" />
Again, this is all driven from the register directive at the top of the page.
Chris