You don't need to go up to 400°F to decompose baking soda into washing soda. Decomposition starts around 122⁰F (50⁰C) and is complete at around 250⁰F (120⁰C)
Depending on the thickness you put in the pan you may want to put the oven around 300⁰F so to speed up the process.
You can safely go to higher temperature as it won't ever be overcooked.
Heating to transform (baking soda) sodium bicarbonate into (washing soda) sodium carbonate does remove moisture but also removes carbon dioxide :
When sodium bicarbonate (NaHCO~3~) is heated, it undergoes a decomposition reaction to form sodium carbonate (Na~2~CO~3~), water (H~2~O), and carbon dioxide (CO~2~).
The reaction can be represented as :
2NaHCO~3~ + heat → Na~2~CO~3~ + H~2~O + CO~2~