possible mild posterior fossa atrophy?!

It was in 2000. Your doctor would have seen it. If he/she was concerned about it, they would have addressed it.

When you have a CT or MRI or ultrasound...the radiologist tells you all the things they can find that may be off or may be fine. They aren't trained in the specifics. For example, I get 3 MRI's a year because I have multiple sclerosis. The radiologist isn't trained to find every lesion or know how to. My neurologist always reads the scans himself because he knows what to look for...and often finds lesions the radiologist missed. This isn't uncommon.

Depending on your age, there are often signs of the spine aging as well....and it's normal.

But, it you're worried 11 years later even though you don't seem to have had any problem with your back since then, go see a doctor about it and get another scan done.