I strongly suspect that the subsequent install of the original OS will not activate - that then is not a qualifying OS that you could use to do the Win10 upgrade.
MS has stated that hactivated or non-authentic systems will upgrade to Win10, but they will not be activated.
Remember the hubbub about pirated versions ??2. Installation and Use Rights.
a. License. The software is licensed, not sold. Under this agreement, we grant you the right to install and run one instance of the software on your device (the licensed device), for use by one person at a time, so long as you comply with all the terms of this agreement.
Updating or upgrading from non-genuine software with software from Microsoft or authorized sources does not make your original version or the updated/upgraded version genuine, and in that situation, you do not have a license to use the software.
I suppose you might be able to rearm what is then basically a trial install, but I'm not certain.
It's also unclear what happens when the trial period, or extensions of it, expire.
Warnings, then no boot?
I only know that some features of Windows 10 (personalization for example) are not available if the install is not authenticated and activated.
The new licensing agreement closely follows the old licensing agreement. Mostly, the wording has been simplified and some EULAs are consolidated into one. MS will most likely issue new EULAs as Win10 and other MS programs (Office 2016) mature.