Shortcode to insert default text and change one word throughout it?

You can use somthing like this:

function my_custom_shortcode( $atts ) {

    // Attributes
    $atts = shortcode_atts(
            'word' => 'example', // the word which will be replaced
            'part' => '1' // for identifing the different texts

    // only if a word exists
    if ( isset( $atts['word'] ) ) {

        // see if part="1" was inserted in the shortcode...
        if ($atts['part'] === '1') {

            $text="Lorem ipsum dolor sit amet, ".$atts['word'].' sadipscing elitr, sed diam nonumy eirmod tempor invidunt ut labore et dolore magna '.$atts['word'].' erat, sed diam voluptua. At vero eos et accusam et justo duo dolores et ea rebum. Stet clita kasd gubergren, no sea takimata sanctus est '.$atts['word'].' sit amet.';

        } else { //... if not use this text

            $text="At vero eos et accusam et justo duo dolores et, ".$atts['word'].'.';


        return $text; // return the text

add_shortcode( 'textblock', 'my_custom_shortcode' );

We use a shortcode with 2 attributes. One attribute is the word which you want to replace. The second attribute (part) is for identifing which static text you want to show.
As you see this will only work with static text which you entered in the shortcode before.