score:2

Simply change numericSymbols.

Snippet:

Highcharts.setOptions({
    lang: {
        numericSymbols: [ "A" , "B" , "C" , "D" , "E" , "F"]
    }
});

Example: http://jsfiddle.net/BlackLabel/7ndqujhq/

score:4

Try adding format parameter so Highcharts won't do its own formatting.

yAxis: {
    labels: {
        format: '{value}'
    }
}

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