Baking powder is baking soda plus an acid (often tartaric acid, but citric and malic are probably also used in some products). They will start to react and produce CO2 as soon as you get the baking powder wet, whereas baking soda produces CO2 by thermal decomposition.
If you want to replace baking powder with baking soda in a recipe then you should probably supply some acid to compensate (e.g. a squeeze of lemon juice, if that fits the flavour profile). The other way round is trickier; it might work in some recipes but not others.