The short answer is, we don't know for sure because Microsoft hasn't announced anything yet (at least as far as I'm aware).
However, the whole reason Microsoft is offering the free transition with a time period, is because they WANT everyone (or as close to as they can get) to migrate to Windows10. They want program developers to embrace their vision for universal apps and developers will only bother to do that if enough of their market supports it (i.e. runs Win10). I really don't think Microsoft are trying to trick anyone here, they really need programs... and for that they need fast Win10 takeup.
So while I don't know for sure, I'd be surprised and disappointed if Microsoft didn't make it easy, including providing ISOs. You...