Power Query Transform a Column based on Another Column

Question

I keep thinking this should be easy but the answer is evading me. In Excel Power Query, I would like to transform the value in each row of a column based on another column\

Answer 1

You can't transform the existing column with such a step. Use a new column, do the transform, then delete the existing column. What's the big deal? In Excel you would not expect a formula to change the value of a different column, either. In Power Query, the result of a formula is also stored in a column, and that cannot be the column that provides one of the formula input values.