What Ted has said was done recently w/ a client of mine in Edmonton. The Win8 Upgrade burnt to DVD was used on a 64-bit machine. The machine HAD a 32-bit Vista on it. The drive holding Vista was deleted & formatted. Win8 was then installed to the (same) drive w/ no installation or activation issues what so ever. Nor any since the install.
Still, one sure way not to have any such concerns would be to use Win8 non-Upgrade versions (discs). System Builder versions could, also, be used but, those get no support from Microsoft. They use PPU Licenses and being the equivalent of OEMs don't get MS support... only support comes from the installer... a System Builder or manufacturer.
I am a certified MS Licensing Expert and even I can't explain or understand why clean installs from Upgrade media seem to be successfully doable. And that's even w/out cheating w/ the old 'double pass' trick, like people used w/ 7. I did not expect it would work because you couldn't (anymore) delay activation or delay entering a Key. Plus, I beta tested Vista and 7 for MS. We were very clearly & emphatically told not to go from a beta of either to RTM or GA. Yet, w/ 8, I have had MS people say could (now) go from RP to RTM or GA. You can see why I'm at a bit of a loss to know what to say regarding all these questions & concerns. Hell, even, using the Win8 Upgrade to an existing OS, when you tell it to delete, format & "Keep nothing", how is that not amount to a clean install. No kidding it's confusing.
At the end of the day I'm not even going to have any 1st hand experience w/ these scenarios. I'm going to be dropping my RP & using a Full Version Windows 8 Pro. Plus I'm overhauling my tower. 8 will be going on a SSD, there will be no more dual-boot to 7... I will be creating VMs for XP, Vista & Windows 7 on the Windows 8 host machine using Hyper-V. One bonus to this... no worry about 8 killing my Windows 7 license.
I need to have the legacy OSs on tap for client support but, no further need for a physical Win7 machine. I haven't even used 7 since 29/2/12, except to go keep it clean & updated periodically.