Will Microsoft Office for Mac 2011 be affected by an upgrade from Yosemite to Sierra? I would like to know if it is safe to upgrade from Yosemite to Sierra, with Microsoft Office for Mac 2011 (version 14.7.1) installed. Will I have to reinstall it again, or is it safe and there are no worries?
What I can tell you is that Microsoft Office for Mac 2011 does indeed work in macOS Sierra. 

I've also just done a check on its Roaring Apps page and up to version 14.6.8 was confirmed as working in macOS Sierra. While this is not the version you quote (14.7.1), Microsoft typically provides updates to improve compatibility with the OS, so your more recent version will work fine.

As to whether you will have any issues because you're upgrading directly from Yosemite to Sierra, no-one can give you a 100% guarantee as every system has its own quirks (depending on configuration, installed software, connected hardware, etc). However, your current installation of MS Office should still work fine after the upgrade.