If you transition your installation of Windows Server 2012 Essentials to Windows Server 2012 Standard, you should still install Update Rollup 1 for Windows Server 2012 Essentials (KB2781267).
Microsoft recommends that you install this update rollup to benefit from the fixes and new features that it provides. However, you are not automatically offered Update Rollup 2781267 from Windows Update if you follow the steps that are provided in the following Microsoft TechNet topic:
This is because this update rollup is currently targeted only at installations of Windows Server 2012 Essentials. The update rollup installation is not part of the license transition process to Windows Server 2012 Standard.
To install Update Rollup 2781267 on the server on which you transition to Windows Server 2012 Standard, follow these steps:
Review the update rollup release notes to make sure that you plan to install the Update Rollup 2781267. To do this, click the following article number to go to the article in the Microsoft Knowledge Base:
2781267 Update Rollup 1 for Windows Server 2012 Essentials is available