<?php

add_action('pre_update_option_theme_mods_' . get_option( 'stylesheet' ), save_header_image_as_option);
add_action('switch_theme', 'keep_header');

function save_header_image_as_option()
{
    $details_before = get_theme_mods($blog_id);
    update_option("previous_header_image_saved", get_theme_mod('header_image'));
    error_log("before theme change" . json_encode($details_before), 0);
    return $details_before;
}

function keep_header()
{
    $details_after = get_theme_mods($blog_id);
    set_theme_mod('header_image', get_option("previous_header_image_saved"));
    error_log("after theme change" . json_encode($details_after), 0);
    return $details_after;
}
?>