Is it possible to unzip a .xlsm file without corrupting it?

Question

Hello.
I have had some success at actioning the following sequence of steps over a .xlsx file:
1) Rename it to .zip.
2) Unzip it.
3) Make changes across the unzipped XML files with a search and replace tool.
4) Rezip it.
5) Rename it back to .xlsx.
6) Open it up in Excel, without loss of integrity (and with the replacements from the search and replace tool intact).
Does anyone know if this is possible with a .xlsm file? If I eliminate step 3) (i.e., in order to round-trip without making any edits, just to prove that round-tripping works), Excel tells me that the file has been corrupted when I try to open it up.
While on the subject, does anyone know of a tool or method of automating the above 6 steps, so that I can action it over all my Excel files?
Many thanks.
Keith

Answer 1

The .xlsx and .xlsm files behave the same in this respect. 
I suspect you had an error in your process.  In particular, make sure that no new files were created while making changes and that the changes were made properly.
You can use many zip/unzip tools, including Java's jar tool (cross platform, client or server).
Example:
Start with a spreadsheet such as updateTest.xlsm.
Create a subdirectory such as updateTest and cd to that.
Run
  $ jar xvf ../updateTest.xlsm
Then run
  $ jar cvfM ../updateTest01.xlsm *
The above example leaves all the xml files in the subdirectory and the resulting updateTest01.xlsm opens in Excel without warnings.
Note: the “M” option for jar is essential to avoid adding a manifest. 
The “v” option is optional.
It is possible to modify the xml files between the above two steps, but again, make sure there are no new files and make changes properly. 
The file [Content_Types].xml tracks the expected files, so these and only these should be present.
For changing the content, here is an excellent article: Editing Data in an Excel 2007 Open XML File with VBA (http://msdn.microsoft.com/en-us/library/dd819387(v=office.12).aspx)
In summary, you can edit numbers in your xl/worksheets/sheetX.xml files, but strings are centralized in xl/sharedStrings.xml. 
Cells with strings include an index into the sharedString.xml table. 
Note that there is a bug in the above article: any new strings must be added to the end or else they will throw off the indexing for other references.
As a hack, you can include dummy strings in your spreadsheet and then update sharedStrings.xml.
Note that with the java.util.zip package, you can create the files and zip files programmatically. 
This could be better for large numbers of updates.  See
http://java.sun.com/developer/technicalArticles/Programming/compression/.
 Other tools and approaches can be used as well.