I have seen this too many times to count.  "Incorporating Micromath" will eventually drop from the title.  The CONSER Cataloging Manual 6.1.7.d is pretty clear on how to consider this case:

Occasionally, when the title of a serial changes, the earlier title is included on the piece with the new title to alert readers to the fact that this is the same serial. This also occurs when one serial absorbs another and the publisher wishes the reader to be aware of this fact. After a period of time, the earlier title is usually dropped. For this reason, omit the earlier or absorbed title from the title proper (AACR2 12.1B1), but record it in linking entry field 780.

On piece:
International gas report, incorporating World gas report

In record:
245 00 $a International gas report.
780 05 $t World gas report {2nd ind. 5 = Absorbed:}

Steve Shadle/Serials Access Librarian  *****  shadle@u.washington.edu
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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
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> Box 18000
> Pensacola, FL 32523
>