Is there a source (on line preferable) where I can find some
statistics the probability of a hurricane-force storm's
probability of increasing/decreasing on the Saffir-Simpson scale
(Category)?

I recognize that there are a multitude of factors that affect a storm,
and I'm not trying to create a pure forecast simulation model.

What I'm looking for is primarily "high-level" probabilities. For
example, if a storm is increasing in power and reaches a Category 4,
what are the overall odds it becomes a Category 5? Conversely, if a
storm is moving "down" the scale and has gone from a Category 4 to a
Category 3, how often does it then move back "up" the scale to a
Category 4?

Any help or information, greatly appreciated.