When you migrate SharePoint 2007 to 2010, it sometimes happens that the UI gets ‘stuck’ in v.3.0 mode. When you move a database from 2007 and do an attach upgrade, you have a choice to use the -UpdateUserExperience flag when you run the Mount-SPContentDatabase command. This is usefull so that you don’t have to manually update the Visual UI, but this flag can be dangerous as well.
I have used it and sometimes, it causes most sites UI to be upgraded to v.4, but not all. On the not upgraded, the option to upgrade the visual UI is gone which puts you in the spot a bit.
For this occasion, I created this short but effective snippet:
$SPSite = Get-SPSite http://MySiteCollection
foreach ($SPWeb in $SPSite.AllWebs)
$SPWeb.UIVersion = 4;