HELP -- just lost 8 hours of work. I don't understand how file saving works

Also, when using Auto-saved versions, you may see ones named something like "Record Performance + 6 redos". The "N redos" part means that if you pick that state, the History panel will have N states that had previously been undone. You can then click on any of them to return to that state (or hit cmd-shift-Z to redo each change one-by-one).

Sorry to hear you lost work, though I'm hoping the work is actually still there — it's almost impossible to lose work unless your hard disk is corrupted, or you save over existing Photoshop or Illustrator artwork (from within those apps). In Character, everything you do can be undone (or redone).

You are correct that doing File > Save Project As will create a new project that is identical in content to the old one, with the exception that the history (in the History panel) will be cleared out.

Maybe you accidentally jumped to a history state from eight hours ago? If you did the Save Project As in that state, indeed the new project would have none of your recent work, but you should still be able to open the old project and get back to your latest state — the history is retained even after you quit. If you ever hit Save, those states will show up in the top of the history panel. But even if you didn't, you can get back to any state using Auto-saved versions, which are shown by checking this option in the History panel:

If you go back to the original project and look in the History panel, what do you see?