All styles are stored in the database; thus copying the old stylesheet to the stylesheet directory won't work, because it's stored in the database.
Once it's been installed, there is no auto updating done as it could potentially screw up any customizations you did to it via an overwrite.
In short, you must do any stylesheet updates manually, IF you want them... I for one don't like the new UBBCode stylesheet changes, so I left them out and only applied the new entries.
