If a conference accepted a paper with 3 authors, and the 2 co-authors were mainly reviewers. When submitting an extended version to a journal, should the co-authors be included or they can be removed? Can the paper include new authors?
If the work is extending the original, then all of the original authors should be retained. New authors can certainly be added, however, if they have contributed to the extension.
I would find it kind of fishy if the authors change between conference paper and journal paper. It would be no problem if they differ substantially, but if this is not the case then one might try to contact the missing authors.