Drupal 8 - change view mode programmatically and consider cache

yourtheme.theme

<?php
// If you do not add the cache key, you will get
// interesting results in your live environment
// where caching is enabled

function yourtheme_preprocess_node(&$vars) {
...
    $textBl = $vars['content']['field_page'];
    $delta = 0;
    
    while(!empty($textBl[$delta])) {
      $par = $textBl[$delta]['#paragraph'];
      
      if ($delta == 0) {
        $viewmode = 'special';
        $textBl[$delta]['#view_mode'] = $viewmode;
        $textBl[$delta]['#cache']['keys'][] = $viewmode;
      }
      ...
    }
}