You will not have to use ISO for RTM Windows. You will have to install the Windows 10 update from within the Operating system. This information comes directly from Microsoft Insider Software Development If you want the actual information, not leaked information, then you can read my actual thread that Microsoft Development Group gave me about this issue. The thread is Windows 10 Insider Update
Win 7 SP1 and Win 8.1 with Update >>> Upgrade via Windows Update Win 7/8/8.1 RTM >>> Upgrade via ISO only
Just upgrade from leaked ISO if its not activated / wont phone activate just restore prior windows then wait until it pops up on Windows Update.. No loss.