Writing a journal paper based on work already published in conference paper

Question

Im in the Applied Computer Science field (deploying technologies in different areas of society). I published a conference paper last year on one such case study and wanted to reuse this data to create a longer, journal paper about the socio-political realities and broader societal reflections on this work (which was not appropriate for the conference audience).

Im wary of self-plagiarism, so the question is: is it possible to take this same idea and publish it in the journal, if the quotes and participant comments are carried across, does it invalidate my claim to this being unpublished work? (Both SAGE Journals and Elsevier have a journal submission policy that only unpublished writing such as thesis, conference posters etc. can be used for journal writing: Prior Publication: SAGE Journals

If a substantial portion of your manuscript has been previously published, your manuscript will generally not be acceptable for publication in a SAGE journal. However, subject to each journal’s policy, there are certain circumstances where material that has been publicly distributed may be considered for publication.

Does rewriting relevant sections that I will be re-using from the conference publication count as a portion from previously published piece?

Similar answers from Academia SO cover some elements of this question, but my question is about the Findings (Participant Interview quotes) being carried over and the other aspects being fresh.

Does conference presentation of preliminary findings conflict with publication? Using my published work on another paper

Any help with this, or links to resources that help appreciated!

Answer 1

From the SAGE link you provide:

Conference abstracts, posters and presentations

Subject to the journal's policy, manuscripts based on papers that have been presented at conferences may be considered for publication as long as they have not been published and provided that you still retain the rights to the manuscript. The journal editor may review whether the version of your article considered for publication is materially different from the work you presented at a conference and/or whether publication in the journal will enable your article to reach an audience that the conference paper did not previously reach. Prior publication of an abstract or poster presented at a conference will generally not impact the manuscript's eligibility for publication.

A similar policy is enforced by Elsevier (at least AIJ) and IEEE. Prior publication at a conference does not disqualify the content for publication in the journal, but the journal version usually needs to be more detailed or add new information. The standards of a journal are normally higher than those of a conference in terms of completeness of the exposition.

So in general it is fine to extend a conference paper into a journal paper, but you should always check the particular journal's policy, in case it has a different clause that overrides the publisher's policy.

Sometimes, the submission process of the journal specifically asks whether the paper is an extension of a conference paper. Even if it does not, you should make the editor aware of this fact (for example in the cover letter), and cite the conference paper in the journal paper, explaining what the latter adds on top of the former.