Replace full content with an excerpt

Decide about the_excerpt() or the_content() with a conditional: is_singular().

You can use a plugin and filter the_content depending on the current page’s type: archive or singular. But you can use it in your theme too.

add_filter( 'the_content', 't5_replace_content_with_excerpt', 100 );

/**
 * Return excerpt if we are not on a singular post view.
 *
 * @param  string $content
 * @return string
 */
function t5_replace_content_with_excerpt( $content )
{
    if ( is_singular() )
    {
        return $content;
    }
    // remove our filter temporarily.
    // Otherwise we run into a infinite loop in wp_trim_excerpt().
    remove_filter( 'the_content', __FUNCTION__, 100 );
    $excerpt = apply_filters( 'the_excerpt', get_the_excerpt() );
    add_filter( 'the_content', __FUNCTION__, 100 );
    return $excerpt;
}

In your theme find the line where you are calling the_content(). Change it to:

is_singular() ? the_content() : the_excerpt();