Is it permissible to change the intention of fasting the six days from making up for Ramadan to intending the fasting of the six days of Shawwal, and then making up for Ramadan later, due to circumstances of istihadah (post-menstrual bleeding) that prevented fasting the six days of Shawwal with a separate intention?

Since you fasted the days of making up [missed fasts] with the intention of making them up, you have fulfilled your obligation and discharged your duty. You cannot change your intention after completing the act of worship. As for the six days of Shawwal, we hope you will receive their reward through your intention, and some scholars have gone on to say that it is permissible to make them up. Istihada (prolonged non-menstrual bleeding) does not prevent fasting; rather, a woman experiencing istihada may fast and perform all other acts of worship that a ritually pure woman performs. Only menstruation (hayd) or postnatal bleeding (nifas (definition)) invalidate the fast.