Yes, You Can Revert to UI v.3 After Visual Upgrade

One of our customers had done the migration from MOSS 2007 to SharePoint 2010, but something went wrong with the Visual Upgrade. Until the problem is solved we decided to revert back to the UI version 3. But as it goes, when the upgrade has been done to the whole site collection, the options to revert aren't visible anymore. Instead, you need to use the SharePoint Management Shell.

For showing the options on a single site:
$SPWeb=Get-SPWeb YourSiteUrl
$SPWeb.UIversionConfigurationEnabled=$true;$SPWeb.update();

And in all sites in a Site Collection:
$SiteCollection=Get-SPsite YourSiteCollectionUrl
foreach($SPWeb in $SiteCollection.AllWebs){$SPWeb.UIversionConfigurationEnabled=$true;$SPWeb.update();}

You do not need to run either one of the above if you are going to simply revert the site/Site Collection to the previous UI, but if you want to show options again for testing purposes, those are the commands to use.

For the actual reverting to UI v.3, use this for the Site Collection (or modify command to use it for one site only, similar difference as shown above):
$SiteCollection=Get-SPsite YourSiteCollectionUrl
foreach($SPWeb in $SiteCollection.AllWebs){$SPWeb.UIversionConfigurationEnabled=$false; $SPWeb.UIVersion=3;$SPWeb.update();}

In the example above I hid the options again so that no one could mess with the UI versions again until problem was solved and version changed by administrators again.

For more reference on Visual Upgrade, see TechNet Library.