The only catch with a pirated version of Win10 is that they have to stay in the Windows Insider (beta testing) group to try all the new builds/features before others. If that means registering with Microsoft, I'm not sure how many pirates will want to do that.
For the rest of us, even IT Pros, Microsoft is trying to convince us that upgrading is better than a clean install, and that their upgrade tech in Win10 is nothing like previous editions. While I'd still personally prefer a clean install to upgrading a 5yo installation, you may need to complete the upgrade once in order to get your key, and then start over with a clean install.
Subject to change when the final build comes out and people experiment, but so far it sounds like this is the procedure:
Install all Windows Updates for Windows 7, 8.1
Reserve free copy of Windows 10
Download Windows 10 but don't start installation straight away.
Decrypt and extract an ISO file from the download: clicky
Burn ISO or use Rufus (freeware) to create bootable USB. Keep somewhere safe for future years.
Backup important files just in case.
Launch Windows 10 setup from within your existing Windows OS.
Choose to keep everything and complete the upgrade.
Verify that Windows 10 is activated. Microsoft now knows about your PC (motherboard) and has a record of it being licensed for Windows 10 Standard (Home)/Pro
You should now be able to wipe the PC and clean install if you desire, as MS knows about your PC being licensed. Remember, the license was dependant on UPGRADING from Windows 7 or 8 (or even an earlier 10 build as I've just done). You should be able to Skip entering a product key during clean installation or possibly use a shared public key that tells MS to check your hardware. If you haven't previously activated Windows 10 then you get a message that the key is blocked.
This information applies to build 10240 which was released to insiders 5 days ago and seems near-final. Microsoft are not distributing an ISO for this build any more although you can find one elsewhere. If you have a spare PC and you want to test ahead of July 29, install an older Win10 build and then upgrade to 10240 after activating the first build.