Optimizing conference paper solution to be eligible for Journal article
Let the editors and/or reviewers decide whether the new contribution is sufficient. Mention that you extend previous work. It may be wise to be explicit about the novelties with regards to the previous work in the cover letter.

In case the original paper was published in conference proceedings (e.g. not just a talk), it is very important that you cite it. When you do that, there can be no claim that you are attempting to recycle existing material without anyone noticing (self-plagiarism).