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On launch day, you MAY NOT be able to upgrade all your computers!
As most of us know by now, MS will be doing a rolling upgrade of PCs top Win10 lasting up to a month according to MS. Us insiders will be able to upgrade our current machines that are on the fast ring. Don't expect to upgrade all of your computers that are not on the Win10 preview in the house tho. Yes, we may be able to use a USB stick or DVD made from our RTM upgrade to upgrade them, but it probably will NOT activate nor will you get a valid Win10 license. It may be necessary to wait until MS decides that your other computers are next in line and have to use the MS servers to do the upgrade for the activation and license to properly validate the computer. MS will be pushing the upgrades out at their own pace and I doubt that there will be any way to get around this except by getting them all on the Win10 preview version.
This is my own opinion and I have not seen anything by MS to back this up, but be prepared.
From a post here from Mick823: I joined the insider program about 6 months ago and have been getting the updates regularly. When the 10240 version came out I updated one PC via the update route. I then used the ESD file from that installation to create an ISO to make a clean install on a second machine. The installation worked OK but the activation will not work no matter what you try. I had to revert back to the old version via an image and then do the in place upgrade to get a validated copy. It will be interesting to see how you end up with installation media that will activate properly.
Any Official Sources of Windows 10 Upgrade as ISO? - Page 2 - Windows 10 Forums