This is a quick "off-the-top-of-my-head" answer -- if all that has happened
is a simple title change, we follow ALA rules, and leave the same call number,
with a "Continued by..." note on the old title, and a "Continues..." note on
the new.  If there is a merger of two periodicals, but the voluming continues
that of one of the merged titles, then it takes more thought: sometimes the
new title gets a new call number (usually just a Cutter change) but sometimes,
especially if we own only the old title whose numbering is continued, we use
the old call number. That certainly seems to be the practice of most (if not
all) the California State University campusses.  Does that help?  Of course,
I should preface this with the usual serials, "It all depends"!  Let me know
if you want a more scientific answer.
                                     Kathleen Thorne
                                     Serials Cataloger, San Jose State Univ.
                                     kathleen@SJSUVM1