Digital signatures constantly getting invalidated
 

We have Excel 2007 workbooks that we regularly route through a workflow via
MOSS for digital signatures, and previously, this process worked flawlessly.
But now anytime someone signs a workbook, the next person on the workflow's
distribution list gets a message that says, "Signatures cannot be added or
removed because this read-only workbook has been changed" . . . but nobody is
making ANY editorial changes to these workbooks!

The only thing we've done differently is that we added some macros to this
workbook that includes some custom functions that changed the file extension
from .xlsx to .xlsm. Is that what's invalidating the electronic signatures?
This error message also says, "Remove signatures, enable editing, save your
changes, and try again," but nothing in Excel 2007's online help tells you
HOW to re-enable the editing.

If both of these questions were previously answered, please point me to the
right thread. Thanks.