Replacing values in multiple columns based on condition in Power Query

Another way to do it, is the unpivot your columns, and then on the newly created column apply a conditional replace operation. 

= Table.ReplaceValue(
     #"Changed Type",
     each [Value],
     each if [day flag] = "Down Day" then null else [Value],
     Replacer.ReplaceValue,{"Value"}
 )

You can find the details on the conditional replace right here: 

https://gorilla.bi/power-query/replace-values/#conditionally-replace-values

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