This probably isn't the answer you're looking for, but the only solution
that occurs to me is to mark the file as read-only in Windows.
(Right-click, select Properties from the menu, select the Read-only
checkbox, then click OK.) That would prevent Acrobat from overwriting it
with the filled-in version. I'm not aware of anything within Acrobat
itself to accomplish this, but maybe someone else has a better idea.