The reason to upgrade is that you will be left with a soon(-ish) to be unsupported OS instead of upgrading to a brand new OS that will be supported for a long(-ish) while, for free. (If you upgrade grudgingly 2 years down the line when 10 is required for newer hardware / or 7 is unsupported like XP is now, you will fork over $$$$$)
So, upgrade it, if you don't like it, put it back, I do believe the license will not give you an issue. When 10 is released, I'll check that out and post here with the results.
And, like I said, install 7 next to 7 in a different partition using the same license, then upgrade one of them to 10. Back up / image the fresh installs of 7 and 10, then you can always go back.
It's not that I'm being "You MOST USE 10 it's shineee and nuuu" I'm being "Why give MS your hard-earned $$$ later when you can upgrade for free and have your cake and eat it, too?".
If you don't get anything else out of this, consider: The inner workings of 10 are at least slightly more robust than 7 due to advancements in the OS, I would kind of hope. So, why not take advantage of the better engine, the longer support, and then, if you don't like the trim panels and paint job (UI), don't throw the car away -- the engine and frame are still good, use
ClassicShell (which is already compatible with Win 10!!).
EDIT: If you want to go further down that classic trim & paint route, get
UltraUXThemePatcher, which is also already compatible with Win 10.
Guide is
here, with links to some classic themes (7 and XP and Classic {aka Win 2K}).