If you are upgrading to SharePoint 2013, and not quite ready to upgrade the UI, you may want to hide this rather annoying popup bar, since it will continue to appear, even after you dismiss it. A good example would be if you want the stability of SP13 and access to the new Service Application infrastructure.
A simple way to dismiss this message is via PowerShell (SP13 Management Shell). This command will hide the message for 1 year. You can adjust the date range as desired:
Get-SPWebApplication http://portal.contoso.com | % { $_.UpgradeReminderDelay = 365; $_.Update() }
Or alternatively, to hide it entirely right out of the gates for all applications:
Get-SPWebApplication | % { $_.UpgradeReminderDelay = 365; $_.Update() } Get-SPSite -Limit All -CompatibilityLevel 14 | % { $_.ExtendUpgradeReminderDate() }