You can only edit your posts for a certain length of time. Editing is more for correcting a mistake (like .xml instead of .xls). If you have new data to add, you're better off adding another post to the thread, since some people (like me) won't go back and read the older post if they've already read it.
This is not the answer you're looking for, but I've found that sharing workbooks in Excel, particularly across pre- and post-2007 versions is an exercise in frustration. Many features don't work in shared workbooks, and many actions (like inserting rows as opposed to just updating cells) can wreak havoc on the workbook. Crashing workbooks are actually the least of the problems. In my experience, corruption and data loss can also happen, so make sure you're backing up the workbooks from the network.
You'll hopefully find success by upgrading everyone to 2007 and it may work fine for you. However, if it were me, I'd consider a database solution. Excel is not a true mutli-user platform and although Microsoft has created some ability for that, I wouldn't rely on it. If it needs to stay in Excel for some reason, it might be worth asking whether they REALLY all need to edit at the same time, rather than choosing to be notified when the workbook is available and making edits one at a time.
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