A case if published in a journal as 'case report' and in another journal as 'case study'. How ethical can that be. I have found such instances. That's why asking for sources of clarification in any literature or book.
Categorically case report is merely reporting the facts without much of analysis and interpretation. On the other hand, a case study implies not only reporting but involves logical judgment, analysis and critical interpretations to pave the way for serious research.
I believe that it would be much better if you could present the case report and discuss it thoroughly as study in the same paper for comprehensive understanding of the readers.
I don't think it's legal or ethical to do what you suggest. From a legal standpoint, much depends on the copyright agreement that you sign when you publish the first paper. Sometimes you are allowed to reuse that material in any way that you wish but usually you are not allowed to do so. From an ethical standpoint, unless you are substantially adding to the second publication so that it is quite different from the first, plus you provide a note indicating that it is a substantially updated paper from the previous one, you shouldn't publish it again.