Fortunately, AACR2 includes a rule about just this situation.  12.1B1
states

"... If the title proper includes any statement that mentions an earlier
title, title absorbed, etc., that may or may not be gramatically linked to
the rest of the title, do not transcribe such a statement as part of the
title.  Do not give the mark of omission.  Give relationships with other
bibliographic resources in a note (see 12.7B8)."

Here is the example:  International gas report

Note:  absorbed World gas report

(Title appears as:  International gas report, including World gas report.

I think this rule covers your situation. NSDP would follow this rule and
not assign a new ISSN.

Regina  R. Reynolds                     email: rrey@loc.gov
Head, National Serials Data Program     voice: (202) 707-6379
Library of Congress                     fax    (202) 707-6333
101 Independence Avenue, S.E.           ISSN Web page: lcweb.loc.gov/issn/
Washington, D.C. 20540-4160

On Tue, 28 Feb 2006, Shankle, Jean wrote:

> Here is a good one.  I need some help from those experienced minds that
> deal with title changes.  The Association of Teachers of Mathematics had
> two publications.  MT (Mathematics Teaching) and Micromath.  They have
> merged them together, which isn't all that unusual.  What is unusual is
> the new name.  The new name is: MT Incorporating Micromath.  That is
> what it says on the title page and on the table of contents page.  They
> have kept the same issue numbering that they used for MT (Mathematics
> Teaching).  The publisher has told EBSCO that is the new name.  We
> didn't want to believe them, but I now have the first issue and that
> what it says.  Suggestions?  Ideas?  Catalog it has the new name?
>
> Thank you.
> Jean
>
> ############################
> Miss Jean Shankle
> Periodical Librarian
> jshankle@pcci.edu
> Pensacola Christian College Library
> Box 18000
> Pensacola, FL 32523
>

Regina 	R. Reynolds                     email: rrey@loc.gov
Head, National Serials Data Program     voice: (202) 707-6379
Library of Congress                     fax    (202) 707-6333
101 Independence Avenue, S.E.           ISSN Web page: lcweb.loc.gov/issn/
Washington, D.C. 20540-4160