Early this year we submitted and presented a paper on an IEEE conference. However, due to the page limit we had leave out quite a few of our experiments and results. As most the work has been carried out already, I would like to submit an extended version of that paper to a journal. Unfortunately the best fit is a special issue at another publisher.
Now comes the tricky part: What exactly does IEEE's copyright on our previous work cover? It is no problem for me to rewrite the text, redraw the included figures, and add a reference to the conference paper in the introduction. But as the added content comes from additional experiments and measurements, the sections on related work and methodology will still repeat the same ideas and formulas as before.