functions.php

add_filter('woocommerce_currency_symbol', 'change_existing_currency_symbol', 10, 2);

function change_existing_currency_symbol( $currency_symbol, $currency ) {
     switch( $currency ) {
          case 'DKK' : $currency_symbol = 'DKK'; break;
          case 'NOK' : $currency_symbol = 'NOK'; break;
          case 'SEK' : $currency_symbol = 'SEK'; break;
     }
     return $currency_symbol;
}