publishing a conference paper in a journal
In my field this would be not only acceptable, but expected, as the review process for conferences is less stringent and the submitted abstracts are substantially less detailed than a full manuscript. A conference presentation is viewed almost as a necessary step to publication: present the first interpretation of the results, get feedback from colleagues, and incorporate changes into the full-length manuscript. 

My only concern would be if the conference paper was closer to a full manuscript, both in length and in the rigorousness of the review process.