Well, retroactive changes to p tend to be much smaller since most evidence degrades with time.

That's a fair point. So, yes: if p is effectively constant and SP is not, you're right that that's a good reason to keep applying the old SP to old prisoners. I stand corrected.

Also in this case since the crime is attempting violent overthrow of the government retroactive changes in p are almost non-existent, after all a successful overthrow by its nature virtually eliminates your chances of getting punished for it.

So are you saying the SP-setting strategy you're proposing doesn't apply to crimes that don't destabilize the criminal justice system itself?