I would like to know...
a site styled by editing the base.css file in themes may be overwritten if a theme update is applied.

Also, if I were in a site that DID edit base.css, can I copy the css and paste it into the CSS editor within admin panel theme editor then upload a default fresh base.css for that theme and my site will stay "styled"?

My changes should be identical within the css edit as they were in base.css and the changes will be moved to userfiles themes thus not being affected by any theme updates.

Does any of that make sense?