I have a bunch of typical sepia color fading old family photos. I am going to scan then with Epson software at 600 DPI, 24bit colour, Tiff format, unmasking on, original target size, autoexposure off. I want to copy so it is as close to original as possible. If I decide to make changes to the scanned photo later, can I then add unmasking, and other colour histogram changes or is it best to do so at the time of scanning? I want to minimise degradation of the image.
I do quite a bit of old photo scanning for various purposes. I have a similar process as you describe and will sometimes go up to 1200DPI for color if the situation calls for it, however that can generate some quite large files. The only thing I do at scan time is set the white point to yield an image as close to the physical one as possible. Any clean up, shifting or dust removal is done after the matter in photoshop. This ensures the scan is effectively a "copy" of the photo in digital form.
Preventing degradation of the image has more to do with how its handled than the scan. You should be using lint free gloves, make sure your scanner bed is clean, blow the dust off frequently, and put the image right into a proper archival sleeve when you are done with it.