If you don't mind the process: Do the upgrade so that you product key is in MS's system as having accepted the free upgrade, and then revert back to or reinstall Windows 7. If you install 10 down the road (upgrade or clean install with the same product key), your product key should automatically be activated and you'll be good to go.
On a related note: It's really sad that so many people feel like upgrading is such a precarious thing. MS seems to have really shaken people's trust in their product. And for those running Windows 7 (or 8) and concerned about the lack of privacy in Windows 10? Don't worry: MS backported their spying schemes to Windows 8 and 7 users! (And you thought Microsoft nearly forgot about you...
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