[Solved]-Change Highcharts tooltip formatter from chart Object , after chart is rendered-Highcharts

you can use chart.tooltip.options.formatter instead, like

chart.tooltip.options.formatter = function() {
    var xyarr=[];
    $.each(this.points,function(){
        xyarr.push('serie: ' + this.series.name + ', ' +'x: ' + this.x + ', y: ' +this.y);
    });
    return xyarr.join('<br/>');
}

changing tooltip formatter dynamically | highchart & highstock @ jsfiddle

update in new (5.0.0+) versions of highcharts, this can also be done using the chart.update() method

  chart.update({
    tooltip: {
      formatter: function() {
        var xyarr = [];
        $.each(this.points, function() {
          xyarr.push('serie: ' + this.series.name + ', ' + 'x: ' + this.x + ', y: ' + this.y);
        });
        return xyarr.join('<br/>');
      }
    }
  });

changing tooltip formatter dynamically with chart.update | highchart & highstock @ jsfiddle