What is the ruling on prayers whose intention was changed from obligatory to supererogatory due to obsessive doubts – are they invalid and must they be repeated?

These are whisperings (waswas) that you must struggle against and turn away from. Do not pay attention to what Satan casts into your heart regarding changing the intention for a supererogatory (nafilah) prayer, and do not repeat any of the prayers. Do not give any importance to the whisperings that occur to you during prayer, and any thought that comes after the prayer about turning it into a supererogatory one has no effect. You must completely ignore these whisperings.