>From the ISSN perspective, "you can't go back again," as I've had to tell
many a frustrated publisher who changes his title, gets sent a new ISSN,
regrets the change and resumes the first title.  Imagine his chagrin when
he receives yet another ISSN!  Although there have been exceptions (e.g.
cases where there was a typo or other printer's error in the title),
generally an ISSN cannot be resumed after a different title intervenes.
The key titles for both A (1) and A (2) are qualified by date only.  ISSN
catalogers call these titles "flip-flops."  In short, title changes in
the ISSN system go only in one direction:  forward.  With the exception
of the issue of title qualification in both the key title and the 130,
the title change is handled just the same as if it had been A-> B-> C.

Regina Reynolds
Head, National Serials Data Program
Library of Congress
rrey@seq1.loc.gov