Well, I imagine that if they treat the upgrade to 10 as they've treated any past upgrade, where you can create a separate installation disc/drive by downloading an image of the full installer. And either during the Windows 10 installation process or thereafter, you might be able to get the activation key if it turns out that they don't reuse your key from 8 or 8.1. (I'm not sure if they treated "upgrade" installation media differently from "retail/fresh install" installation media for Windows 8/8.1, but there have always been ways to work around that if you need to use an upgrade disc for a full fresh install).
With the move from 8 to 8.1 (and apparently going to 10), you had the (preferred) option of downloading the installer through the Windows Store (with any prerequisites being served by Windows Update) to do the upgrade more easily, but that'd mean you would have to roll back to 8 or 8.1 then upgrade, so every time you want to reinstall, you'll have to roll back, then download the upgrade, then install.
This second option is the option you should expect to use (as they expect the average consumer to go this route), so if your Windows 7 systems came with recovery partitions & allow you to create recovery media, you should probably have that created BEFORE the final upgrade. Otherwise, if you manually installed 7, you'll just have to install 7, maybe activate, then install and activate 10. That way, if creating a Windows 10 installation disc/drive fails, you'll have a backup.
(Again, this assumes they stick to the ways they've been doing things).