When submitting an extended version, is it mandatory to include all authors of the original paper? In my field, it's a common practice to publish a work at a conference first, then extend it (typically with 25%-40% more content) and submit it for publication in a journal.

Often the extension is driven by a subset of the original authors, for example, because the remaining authors lost interest or don't have time to develop the extension. In this situation, the question arises whether it's mandatory to include the non-contributing authors as authors of the extension.

Arguably, the authors have already received the credit for their contribution with authorship of the original paper. On the other hand, their contribution to the original work is still a contribution of the extended work. 

Is there any official guideline of how such cases should be handled?
An extended version of a conference paper supersedes the conference paper. A priori, this means that every author of the conference paper is also an author of the journal paper. In same rare cases it could be appropriate to drop an author (if every part they contributed to gets removed), but as a matter of protocol, I would only consider this on request of the relevant author.

This is different from a subsequent paper building on a prior paper from the same project, where a citation to the previous paper suffices. In such a case, one would not copy over important text anyway.