Is it permissible to fast other days instead of the White Days to obtain their reward, and is intention required for that?

If one misses fasting the Ayyam Al-Beed (the white days), and it was their custom to fast them, then it is recommended for them to make up for them, and by doing so, they will attain the reward of performance, especially if the missed fast was due to an excuse. The intention for making up the fast is required, just as it is for performing it. Furthermore, specification is required in the intention for ordered fasts, such as the fast of Arafah, Ashura, the Ayyam Al-Beed, and six days of Shawwal.