My SQL 2008 R2 eval had expired.  All I had was a Volume License (Open Business) and it showed “no product key needed” when I clicked on “key”.  I downloaded full SQL 2008 R2 (not SQL 2008 R2 SP1, it is not full version, only SP1) from Open Business website (as recommended by a helpful post ,thanks!), extracted to hard drive using Extract Now freeware.  Then opened the x64 (or x86) folder and opened the DefaultSetup.ini file.  The Product ID (PID) was there!  I opend the SQL Install Center on my SQL server, clicked on Maintenance, then Upgrade Edition, put in the PID and it was upgraded to SQL Standard in a few minutes!