In general, these are the modifications required to change a conference paper to a journal paper:

1. Ttile: the new title should be different enough and captures the broader focus (i.e., the extra 30%)
2. Abstract: should include 2-3 sentences on the new material
3. Intro: should include a reference to the previous conference paper and
explains explicitly the new contributions of this paper.
4. Intro: should include 2-3 sentences explaining the gist of
the idea behind the new techniques and also 1-2 sentences explaining the new results.
5. Related work should be updated with the papers appeared after the conference paper and before the journal
submission
6. The experiment section should be extended to include the evaluation of the new material
7. Conclusion should include a couple of sentences summarizing the new material.
8. Include a "difference document" (a sample is attached)

For your specific paper, you need:
1. Use Sigmod-rejected experimental-methodology and redo experiments
2. Add one section with 3 new variations (each as a subsection)
3. Conduct experiments for the new variations.