I'm scared to update to the newest version (I'm a couple versions behind I think). I have a many-hundreds of pages gamebook I've been working on for years, off and on. Can I get a comforting 'ok' from the community that I can update without destroying my work? Again, this is in Gamebook mode, which I fear none of you use or do much (if any) QA for, as that mode has fallen by the wayside... which is why I ask.

Should I just stay with my older version to be safe? And if so, will I be able to post the game here in the future (probably a couple years from now, based on my slow trajectory)?

Make a copy of your game first (because opening in a new version could change it, and it does save automatically). If you have a problem re-install the previous version. They are all here, back to 4.0.5:
http://textadventures.co.uk/quest/desktop

Shouldn't you switch to the highest version in any case? At the latest, when the game is uploaded, the game runs on the server under the latest version of Quest and then version errors could occur. Or am I wrong?

But make lots of copys of your games! Best thing is to put them in zips with the version number and the date of the last change in the filename. And if possible distribute them to different storage media.

At the latest, when the game is uploaded, the game runs on the server under the latest version of Quest and then version errors could occur

The core libraries are included in a game when it is published. So a game published with a previous version will still include the previous versions of the libraries. Similarly, the player includes a lot of C# code equivalent to if (version > 570) or similar, meaning that games created with an older version of Quest will still perform like they did under that version.

The player is capable of pretending to be an older version.

It's only when you open a game in a later version of the editor that its version number is updated and it starts using the current libraries.