How to update calculated value?

I added a calculated value to https://jsfiddle.net/a21uj95v/4/, say:
{
"uniqueName": "Number of colors",
"formula": "count(\"Color\") ",
"caption": "Number of colors"
}
Save to json report. It worked.
But next, I updated the formula to 
{
"uniqueName": "Number of colors",
"formula": "distinctcount(\"Color\") ",
"caption": "Number of colors"
}
then, I save the report to Json file again. It doesn't save the updated formula, only the old formula.
Since it's always possible for user to update the calculated value, how can we save the updates?