Install 2.0.1 through wp-admin

I'm using 1.6 Catch Everest Pro already. Why can't I use Appearance/Themes/Install a theme in .zip format/ to install the theme - directly uploading the .zip over the old? The instructions say to enabled another theme, update by installing and switch back to Catch Everest Pro. Shouldn't a theme update work directly with using the .zip in the admin without switching themes? It seems more risky switching themes than updating in-place. Stephen