Is it permissible to change the intention of fasting extra days of make-up for Ramadan to be from the six days of Shawwal, if it later becomes clear that I fasted more than what was required of me?

Fasting the six days of Shawwal requires a specific intention and must be observed on particular days. The extra days the questioner fasted, beyond what was necessary for making up missed fasts, do not count for the six days of Shawwal, because she intended to make up missed fasts, not to fast the six days.