You can't simply swap bicarb of soda/baking soda with baking powder. It is always easier to use the one that the recipe states because:
1. Bicarbonate of soda is 4 x stronger than baking powder. Therefore, you will need to reduce the amount needed (although not necessarily an issue in itself).
2. Bicarbonate of soda requires an acid to activate it. You'll require around 125ml or 1/2 cup of acid to be added to the recipe for each 1/4 teaspoon of bicarb. This will certainly affect the taste and end result. However, do note that many ingredients you may not expect contain acid. Even brown sugar, buttermilk and cocoa powder.