I am a little confused. My office trial on OEM Windows 8.1 just ended, and I think of renewing it. Is it going to be invalid after upgrading to Windows 10? Are current Office programs going to be outdated with Win10's office?
Your 365 subscription should be fine. Worse case you'll just have to reinstall. But if you have a subscription you can just download the installer from Office.com. Best case m, it'll survive the upgrade (mine did doing an upgrade to the 10 TP).