Is it permissible to observe voluntary fasts, such as those on the days of Ashura, Arafah, and the six days of Shawwal, while still having unobserved obligatory fasts from previous Ramadans?

There are three issues here: First: The expiation for delaying the make-up of Ramadan fasts for someone upon whom the next Ramadan has arrived without having made up what was due from them. This expiation is obligatory for whoever neglected the make-up fasts while being aware of the impermissibility of delaying them. The expiation is to feed a poor person for each day, and its مقدار is a mudd of food (approximately 750 grams of rice or its equivalent).

Second: It is permissible to observe voluntary fasts, such as the six days of Shawwal, Arafah, and Ashura, for one who still has make-up fasts from Ramadan due, though it is preferable to make up the obligatory fasts first. It is not valid to change the intention of a fast after it has been performed, as the intention was for a voluntary fast, and it is not considered a make-up for Ramadan.

Third: If someone dies and had fasts due but was unable to make them up due to a legitimate excuse, then there is nothing upon them, because Allah does not burden a soul beyond its capacity.