Better to research it first for support warnings about APFS.
You can revert, but it's more work, and you'd lose anything done in the meantime. Turn TM off before the upgrade, so there's no chance of HS doing full fresh backups. If TM managed to run out of space, it would begin deleting your existing backups. Test all apps in HS. If you run into too many problems, reinstall Sierra, and use Migration Assistant to import your last user backup from TM. But this would all take a long time.
I suggest waiting for other people to kick in suggestions. I've only played with HS a little bit, and since I have an HD system, I haven't played with APFS at all.