[Zope] Question of ZPL etiquette

Hi. I am trying to get a better sense of how others are dealing with ZPL headers in code from original authors once changes are made. For example if a package is ZPL copyright somebody, I typically see a header at the top of most files as well as the LICENSE.txt in the package itself. When changes are made however, it is typical see headers replaced with the header of the author(s) who have made changes and date. A second license will now apply to the modified package with the ZPL license of the original code left to acknowledge authors and copyright of the original product/package. Is that a fair assessment? Is that generally acceptable? Many thanks. Regards, David