Perhaps the question means something else, namely this one: "Is it OK to publish a previously published conference paper also in a scientific journal?".
The answer to that question is "Perhaps". If the conference paper is copyrighted by a publisher, then it is in general *not* possible to re-publish it elsewhere. If, however, the paper from the conference is not published with a copyright attached, then it is most likely OK to rewrite the conference paper and re-publish it in a journal. (But make sure to check this!)
The norm -when converting a conference paper to a journal paper - is to not just take the conference paper and publish it without alterations. Why? Because the conference paper typically is short - as a matter of rule conference papers ARE shorter than journal papers -, and the journal paper can be re-written to such that it is expanded from the conference one, in particular the survey part (where you are supposed to link your work to previous ones through citations - with a resulting reference list that is quite a lot longer than the conference paper), as well as adding more experiments, more detailed theory, such as more detailed proofs, and so on.
Perhaps the question means something else, namely this one: "Is it OK to publish a previously published conference paper also in a scientific journal?".
The answer to that question is "Perhaps". If the conference paper is copyrighted by a publisher, then it is in general not possible to re-publish it elsewhere. If, however, the paper from the conference is not published with a copyright attached, then it is most likely OK to rewrite the conference paper and re-publish it in a journal. (But make sure to check this!)
The norm -when converting a conference paper to a journal paper - is to not just take the conference paper and publish it without alterations. Why? Because the conference paper typically is short - as a matter of rule conference papers ARE shorter than journal papers -, and the journal paper can be re-written to such that it is expanded from the conference one, in particular the survey part (where you are supposed to link your work to previous ones through citations - with a resulting reference list that is quite a lot longer than the conference paper), as well as adding more experiments, more detailed theory, such as more detailed proofs, and so on.
Perhaps the question means something else: "Is it OK to publish a previously published conference paper also in a scientific journal?".
The answer to that question is "Perhaps". If the conference paper is copyrighted by a publisher, then it is in general not possible to re-publish it elsewhere. If, however, the paper from the conference is not published with a copyright attached, then it is most likely OK to rewrite the conference paper and re-publish it in a journal. (But make sure to check this!)
The norm when converting a conference paper to a journal paper is to not just take the conference paper and publish it without alterations. Why? Because the conference paper typically is short - as a matter of rule conference papers ARE shorter than journal papers -, and the journal paper can then be expanded from the conference one, in particular the survey part (where you are supposed to link your work to previous ones through citations - with a resulting reference list that is quite a lot longer than the conference paper), as well as adding more experiments, more detailed theory, such as more detailed proofs, and so on.
Conference papers are usually foundations for the development of journal articles. Unless the conference paper is published with copyright laws binding, the paper can be further enhanced and published in a reputable journal.
For some conferences, the special issue is a collection of selected top papers of the conference to be published in the journal. In this case, your paper is an article (by RG mean) or journal paper. If the special issue is only a part in the proceeding, your paper is only conference paper.
Your question is ambiguous with at least two interpretations. 1. If an important issue arises during the conference that seems to call for treatment in a paper, that's one possibility. In this case, such a paper could be reviewed by the conference technical committee and published somewhere, perhaps in a journal within the same domain as the conference, if the paper is of higher quality, or if of lower quality or of rather limited interest, as part of the conference proceedings, after the fact. 2. Perhaps, and this seems more likely, there is an desire to publish the conference proceedings. If the conference already has a regular journal for this, papers of the conference are often reviewed and sorted according to which are publishable in that journal after presentation at the conference and which are presented at the conference but are not of sufficient quality or broad enough interest to be included in the journal. If the conference is not associated with a (periodic) journal, then usually all qualified papers presented at the conference are included in the conference proceedings, which is usually provided as a CD for attendees or published online.
If you publish it immediately at a journal - you may found such organ with some colleagues, today this is simple, than it is a journal paper (I don't understand why this should be more important than the quality of the text, but this is another question). To bring reviewer at a loss it would be best, neither to copy it as conference paper nor publish in a journal. You should publish your valuable and much promising research results as a chapter in a just printed book of youngest collected research work. Why? Immediately after publishing the book normally is reviewd in a good online-journal, so your groundbreaking research results are well-known in the whole world, and in the mind of the leading experts in your discipline.