Win7 has just entered the extended support phase (security updates only) and support ceases January 2020:
Microsoft Product Lifecycle Search.
Back-ups are always prudent, particularly when upgrading an OS. You never know when you'll select the wrong option and wipe your drive or trip over the power cord half way through.
It would be best to consider Win7 gone if you do an upgrade. Assuming you have your Win7 installation disc and key you'd be able to reinstall - recovering a previously installed version of Windows
may be possible but don't rely on it.
I guess in part it depends on whether you think the system in question will still be in use in 5 years time. Then again if it's free and you can cope with reinstalling Win7 if necessary and MS does a reasonable job this time around then why not?