How can I get the original Sierra installer now that High Sierra is out?
If you downloaded the Sierra installer before using your Apple ID, you should be able to download it again using the Purchases tab in the Mac App Store. If the installer does not show up in that list, try downloading it directly from the Mac App Store.

I do want to point out that it’s quite easy to reinstall macOS in case something goes wrong while updating. Just make sure you have a backup (e.g. with Time Machine) and you should be fine. In case something does go wrong, press and hold the power button to force you Mac to shutdown. Turn it on and immediately press and hold cmd + R until you see the Apple logo. You should now be able to reset your Mac from there.