I've been using ISSN's (International Standard Serial Number) to uniquely identify journals, both to generate article identifiers, and as a parameter to send to CrossRef's OpenURL resolver. Recently I've come across journals that change their ISSN, which has fairly catastrophic effects on my lookup tools. For example, the Canadian Journal of Botany has the ISSN 0008-4026, or at least this is what JournalSeek tells me. However, the journal web site tells me that it has been renamed as Botany, with ISSN 1916-2804. The thing is, if I want to look up DOIs for articles published in the Canadian Journal of Botany, I have to use the ISSN for Botany if I want to get a result. Hence, I can't rely on looking up the ISSN for the Canadian Journal of Botany. I've come across this in other journals as well.
WorldCat's xISSN web services provide some tools to help, including a graphical display of the history of a journal and it's ISSN(s). Here is the history for 1916-2790, redrawn using Graphviz. WorldCat use Webdot, which I've written about earlier. If you view the source of the WorldCat page you can get the link to the original dot file.
The problem with these changes is that it makes ISSN's more fragile. Ideally, the original ISSN would be preserved, and/or CrossRef would have a table mapping old ISSN's onto new ones. The rate things are going, I may have to create such a table myself.