Someone wrote in asking what the rule is for changing dis- to dys-.
Actually, there is no rule for changing one to the other, as far as I know. Instead, the two prefixes come from different sources.
- dis- is a Latin prefix with a number of meanings. It is by far the more common of the two prefixes.
- dys- has its origin in the Greek prefix meaning “bad.”
Whether to use one or the other depends entirely on the word and its origin.