Hi. Why don't you want to change the series order ?
For some types of chart (e.g. stacked bar) changing the series order will change the appearance of the chart. But for other types (eg. line) then changing the series order makes no difference to the appearance of the chart.
And if you have got a stacked bar chart, Excel puts the legend into the same sequence as the colours in the chart itself, which is a good thing.
If you really want to re-sequence the legend, I think the best way is to create the legend as a picture made up of separate text boxes and so on.