Windows 8 RP, How easy is it to upgrade?

I am running Windows 8 RP on two of my computers and am loving it compared to Windows 7 (don't argue about it haha, I hated the Developer Edition Too...) and am hoping to upgrade it to the final paid version when microsoft release it. I was just wondering though how easy it is to upgrade. I just read something that says you have to prove that you have run a previous version of windows to be able to get the $39 upgrade. I am British by the way, so please don't talk about the American Upgrade (if there is a difference.) This has worried me, since I don't know what data I have to present to prove this, I have no disks since the previous builds were OEM versions and one machine does not have the genuine Microsoft Serial Key Sticker since it was accidentally destroyed when I replaced the case and I worry that it doesn't still have a reciept since it was from so long ago. On my laptop I have the genuine sticker. I take it this is the type of thing they need to prove its genuineness and what do I do about the one that has no proof even though it is genuine. Grateful for any help.
 
It may work just like he windows 7 upgrades did; just install a fresh copy of windows 8 but don't put the key in. Wait until you're in windows then put the key in under system properties.

If that doesn't work I'd say do a clean install, skip the key, stick the disc in and do the upgrade install.
 
It may work just like he windows 7 upgrades did; just install a fresh copy of windows 8 but don't put the key in. Wait until you're in windows then put the key in under system properties.

If that doesn't work I'd say do a clean install, skip the key, stick the disc in and do the upgrade install.

But unless I want to pay the full price - which I CANT afford, I need to prove that I used Windows XP MCE and 7 before hand!
 
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