Can I update Windows server 2012 standard to Windows server 2012 standardR2 without a license update?
Windows Server 2012 and Windows Server 2012 R2 are two distinct products.
Thus, each require a separate product key to be activated.
If you have a valid product key for Windows Server 2012 R2, you should be able to upgrade your server.
Here are more informations regarding upgrade path for Windows Server 2012 R2 : http://technet.microsoft.com/en-us/library/dn303416.aspx
Hope this helps !