My wife is planning on building herself a computer in a few months, and has been factoring in a full licence for Windows 8 Pro ($229).  But I was just reading through the major Windows 8 thread from last year, and there were some instances of people being able to use the upgrade licence by installing it over the Windows RTM installation.  By my calculation, she could buy a Windows 8 Pro upgrade licence for $50 (before 31 January 2013), and then use the Windows 8 RTM + Windows 8 Pro upgrade key to install her OS, and save herself $179.  Are there any fishhooks to that approach?  Does she have to activate the Win8 Pro upgrade key before 31 January, or just buy it before then?  And will the Windows 8 RTM still be able to be installed in a few months time?