upgrade.php

<?php

add_action('after_switch_theme', function(){
    // Set a flag indicating a new installation
    set_theme_mod('theme_version', '1.2.3');
});

// Upgrade routine for backwards compatibility with pre-existing home screen setup.
add_action('init', function() {
    // Check if the update logic has already been executed
    if (version_compare(get_theme_mod('theme_version'), '1.2.3', '<')) {
        // This is either a first run or a theme update
        set_theme_mod('is_backcompat_home_screen', true);

        // Set the flag to indicate that update logic has been executed
        set_theme_mod('theme_version', '1.2.3');
    }
});

add_filter('template_include', function($template){
    if(get_theme_mod('is_backcompat_home_screen') && is_home()){
      $template = locate_template(['index.php']);
    }
  
    return $template;
});