I want to upgrade from 7 to 10 so that I won't lose the chance of upgrading my laptop to a newer version of Windows for free, although I cannot practically make this change until an important piece of software I use daily for work will start supporting Windows 10, which might take a couple of years from the looks of it. Therefore I have to revert back immediately. Will I get to keep my Windows 10 license if I do this, and maybe upgrade when the software I use fully supports the OS? I have a retail key (not OEM) and I've only ever used it on one PC.
Also, do I have to use the built-in revert system or can I just flash back a previous backup image of my hdd?
Yes. After the initial activation you have a permanent digital entitlement stored on Microsoft Activation servers. Subsequent installs of the same edition and version of Windows 10 on the same computer will retrieve the digital entitlement for activation.You can go back to your previous OS any way that you choose to. If you clean install it from install media, however, you may be required to provide the product key for it.
Thanks for the answer!
As you have a retail key, use showkeyplus to extract key from registry before you update if you do not know it.
You can certainly restore an old image backup, but recommend you make an up-to-date one. Actually (IMO) this is much better than reinstalling Windows 7 from scratch, as it can take an eternity to do the zillions of updates that happen when ypou reinstall.
Re. old software - you could run Windows 7 in a virtual machine, or create a dual boot pc (but you would need a separate licence, or accept you have to periodically reinstall Windows 7 every so often (you can rearm the trial licence legally up to 4 times giving you up to 120 days each time you reinstall).