IEEE journal name/proceeding title
I remember in the older version of IEEE style, when exporting to Bibtex or create Bibliography in MS Word, the journal name would automatically change to the abbreviation, such as "IEEE Trans. on Inform. Theory" instead of the full name "IEEE Transactions on Information Theory". Also, the conference proceeding title will automatically have "Proc. of" as the prefix. However, it seems like this function no longer exists in the new IEEE style. Is there a way that I can modify the style file to make this function back?
IEEE still uses journal abbreviations when the journal abbr. field has content in Zotero. That has always been the way it worked.
I don't know about "Proc. of" - I'd say that should be part of the publication title rather than automatically added. I'd be surprised if the style actually used to do that.
The reason I asked this question is because I have a library which was created in Jan. 2011 and the exported Bibtex at that time (I think that was version 2.1) is different from the Bibtex exported from the current version. At that time I can get the Bibtex with abbreviated journal title and the prefix "Proc." in the conference proceeding name, which I can not get with the current version. I am sure I did not change anything in that library since Jan. 2011.
Zotero indeed doesn't export the journal abbreviation to bibtex - I don't know for sure if that has changed (though I doubt it), but it is my understanding that bibtex handles journal abbreviations with lists and it would thus make little sense to export them.
I can't tell you what happened with your old bibtex export, my best guess is that you misremember something - like changing the items in Zotero or in bibtex. You can look through the commit history since Jan 2011 if you don't believe me:
https://github.com/zotero/translators/commits/master/BibTeX.js
I had a quick look and there really doesn't appear to be anything that would have caused the changes you describe.
Probably I changed all of them manually at that time. It's been a while and I just don't remember I did that before. Thanks.