For a detailed analysys of whether continued writes of 0xFF data to an already erased FLASH byte will lead to aging of the byte you really need to communicate to the manufacturer of the chip and see how the chip was designed. That point made let me comment...
no 0xff writes to erased flash

- Some FLASH memories are designed with an intellegent write circuit that will prevent the application of the write voltage to the FLASH cells in the case that the cell content is same as the new data.
the data is different

thanx,

Erik