If you don't have any luck getting MS to let you re-upgrade for free, and you really want to use Win 10 and not Win 7 again, then you can install Win 10 and use it without activation. Simply download a Win 10 install from the MS web site using their Win 10 Media Creation tool (available from MS web site) and save it to a CD or DVD or USB drive, and use it to boot up and reinstall windows 10 on the crashed machine.
When you install, "Skip for now" the activation step - I think it'll ask you twice during the install. The only limit you have with a non-activated version is you can't customize your windows experience, and there will be a little nag watermark at the lower side of your desktop. So in theory you won't be able to change your desktop background theme by right-clicking - so-called "customization." However, if you have an image your want to use for background, you simply navigate to the image, right-click on it and select "set as desktop background."
My experience with Win 10 has not been great. Besides all the privacy invasions it enables, updates have been rather troublesome sometimes.
Not being sure about changing from Win 7 to Win 10, I read an article about installing Win 10 without upgrading or activating (see: http://www.windowscentral.com/you-do-not-need-activate-windows-10 ). All you lose is the ability to "customize" your Win 10 experience -sort of - as the article points out.
So I tried that to give Win 10 a spin. I created a dual boot (Win 7 and Win 10) hard drive (using methods described in another article, see: http://www.technotraps.com/how-to-install-dual-boot-windows-10-along-with-windows-7-or-8/ ) and tried Win 10 for a little over a year, right up until this last major update - the Fall Creator's Update. What a disaster that was! You may have read here at CNET about all the problems people had with it, lost picture files, etc. In my case, that update completely crashed one of our PC's which, by the way, HAD been upgraded from a Win 7 to a Win 10 license beyond salvation. There was a big big problem with graphic drivers apparently, but they didn't figure that out for a week or so - warning: uninstall graphics drivers before installing that WIN 10 update! Anyway. on that PC I just reinstalled Win 7 with the original OEM license (using a system image I had created before the upgrade to Win 10 just in case) and gave up on Win 10.
If MS complains about my doing that, my response will be you screwed up my PC to the point it was unusable with your !@%# Fall Creator's Update update so be thankful I don't sue you. Of course, they probably have language in their license that protects them from damages resulting from all the negligent programming they do.
On my dual boot Win 7/Win 10 PC, I just uninstalled Win 10 and said good riddance without even trying the FCUpdate.
On one of our other PC's that was upgraded from Win 7 to Win 10, the FC'sUpdate installed without any trouble, except that it basically took a whole day with multiple restarts and downloads and so forth. For some reason that PC had a different graphics card in it whose drivers didn't conflict with the update.
My mother-in-laws laptop completely crashed during the FCUpdate, but I saved it by restoring it to a prior-to-the-update restore point. I think she had turned it off in the middle of the update not realizing what the consequences might be. Alas.
I wish I had a dime for every hour I wasted in front of my PC's doing fixes for screwed up MS updates over the years.
Personally, I was underwhelmed with Win 10 and still find Win 7 reliable and useful. But that's just me.
Good Luck!