What I did (for several pc's), downloaded the $40 pro upgrade, got the activation code(s), made usb installation stick, then installed w8 to some spare blank & formatted hd's I had laying around.
Thus not touching any of the original Windows 7 installations & keys, got several non-oem w8 pros at the low price, for testing out and evaluating some potential future swap from Windows 7. & installed Classic Shell immediately, 'metro' now relegated to just a detail that may prove useful or not.
btw, $40 pro upgrade also works with w8 preview installation as basis.
General impression (after using all the w8 previews + the last version), nice os, fast & stable, compatible with everything I've thrown at it, but I see no need to rush to it, my Windows 7's are just as good for daily use (& metroless!). Oh yeah, there's one big w8 improvement ...the UP arrow in explorer is back!!