Miranda was about evidence that could be used against an individual and it's admissability in court. Being able to assert that evidence used to convict one of an act which is, in fact, a crime should not have been admitted and then further showing that the admission could reasonably change the outcome of a case is certainly an "appeal issue", and is quite a different matter from "SCOTUS determined that the offense for which you were convicted is not a crimt".
What I am referring to is a situation when an individual is convicted of an act which SCOTUS subsequently determines was not a crime.
If we're talking about overturning long-standing convictions, then we're probably not talking about appeal. We're talking about collateral review.
Habeas petitions after all appeals are in and done. Ones where the convicts haven't exhausted their appeals are a different matter.
Which exactly do you mean? The retrospective collateral application of a substantive constitutional criminal law decision is a highly technical area on which con law professors currently have broad-ranging disagreements.
If you're talking about whether the decision will affect cases where the convict has
not exhausted all appeal angles, then I agree, they'll have a good shot at a nice new appeal or other direct review.
But remember this case can only directly invalidate the specific federal (DC) laws under challenge. How many convicts on the DC law with the case still open are there? The decision may state new grounds to invalidate other laws - and thus federal cases that are not yet
res judicata may make use of the new "rules" on appeal. The confusing issue of retrospective collateral effect of a substantive change in constitutional law (which would take pages to get into, especially since not everyone agrees) doesn't even come up until some other case changes the rule under which an established conviction without remaining appeal happened - i.e., successfully challenges that particular statute. It'll be a decade of direct challenges to statutes, not a sudden flood of
habeas actions.
If you want to read up on the mess that is retrospective collateral effect, start with
Teague v. Lane, dig around for criminal cases since 1989, continue past where you think you understand... and when your head explodes, you've got it!
Also, if the decision is really going to have significant, immediate impact (which it almost certainly won't until later cases on other federal laws and until a decision touching on the 14th A. question is made), one could even expect the Court to flat out state the retrospective impact, which they can do and have done before.