If you are planning to upgrade your Windows Server 2012 to the Windows Server 2012 R2 Preview (and you are not planning to spend some time reading the documentation), here are some tips that can help you make this more quickly, and (hopefully) without errors:

• CREATE A FULL BACKUP! (before proceeding with upgrade, of course)
• you can only upgrade Windows Server installation which is installed onto physical disk (i.e. you cannot upgrade Boot from VHD(X) installation, as you will get error saying this in compatibility report before upgrade starts) (note that you still can upgrade Windows Server installation in virtual machines, though)
• make sure that you have enough disk space available (the compatibility report says that at least 42609 MB of free space is required)
• make sure that your applications (and drivers) support Windows Server 2012 R2 (unfortunately, site with the compatible apps and documentation (as it says inside the upgrade wizard – http://go.microsoft.com/fwlink/?LinkId=243105) still isn’t alive, so you can’t check your applications there)
• make sure that you have enabled page file on C: drive (I’ve had one on D: only, and the upgrade kept failing until I’ve create one on the C: drive also)
• make sure that you don’t go online and install updates during installation (as this will probably make the upgrade fail, saying that it cannot open Compatibility report)
• remember – if you get error during upgrade, you can always check upgrade log files located in “$WINDOWS.~BTSourcesPanther” (or “%WINDIR%Panther”) folder

I will update this list as something new comes up (also, feel free to comment with your upgrade experiences).

Thanks for reading (and good luck)!