well, actually, dual boot should be fine if you, as stated, buy another license.
i had a reservation for w10 but cancelled it when i found out there'd be an iso available, supposedly, on the same day as the release. that would allow for a clean install.
one could, presumably, use their existing disk to fresh install whatever w/version they currently have and use the iso on another partition. or, they could resize their partition to free up space for a w10 partition. they'd have to sort out the activation on running things, though, as that would involve the licensing issue. i'd assume that, as long as you didn't try to use the old system, you could activate the w10 system.
there really shouldn't be any rush as there's an entire year allowed to watch others sort this kind of thing out before jumping in.
BabaG