How to extend and convert a conference paper into a journal submission?
This varies from journal to journal. For example, in some subfields of CS (where this comes up all the time), the informal rule is that the journal version should have at least 30% new content (including new experiments, new algorithms and so on), and this new content should be identified clearly in a cover letter. In other subfields (like in theory) the expectation is that all proofs will be presented in full detail (no sketches), but new results are not necessary. 

So it depends greatly on the subfield and the journal. As always, when in doubt, ask the editor of the journal, or colleagues who've submitted there before.