This is a hybrid in which all pieces share a common series title, but each also has a unique title. Usually, all pieces are sequentially numbered and dated. As an example, the Montana Bureau of Mines and Geology publishes bulletins, open-file reports, special publications, etc. While these are all series, each item in the series is unique as it has a different title, author and subjects. For a series, it is important to include all elements required on a title page, as well as a consistent series title and numbering scheme.
Readers find it confusing to have several titles for essentially the same publication.
Old and new titles may end up some distance apart on library shelves, if filed by title. It costs more for libraries to create catalog records for each title, and show linkages between them. On the other hand, a significant change in scope or purpose is often best communicated through a title change. If you do change the title, be sure to note the change in the first issue or edition published under the new title. By the way, it is wise to avoid any reference to frequency in the title.