Technically, you should update the offset to the central directory in the Zip footer, along with the offsets to each file header in each central directory entry. If you don’t, the zip file reader has to apply some heuristics to locate the central directory; not all readers implement these heuristics, and those that do won’t always be robust.

The “unzip” utility can be useful as a sanity check; run “unzip -t” to test the integrity of the file.