If, as you say, material can be reprocessed at its retest date, what new retest date should be given to the reprocessed API? If retest is five years, can you reprocess at five years, then wait another five years to the next retest date?

Yes. Chemistry is chemistry. If you’re going to do a physical manipulation then “it depends”,
comes into play. If you’re going to repeat the original process of crystallization/purification there’s
no reason to expect to see something different. If you get something different, then you have an
out of control process. If you know your chemistry, and the material was good for five years the
first time, it should be good for five years the second time.