The ASCII 2DA reader doesn't care about the length of the field. Only that the cells are separated by a space or TAB character. So changing the length of a field doesn't destroy anything. Adding a space ("2745" -> "27 45") would break it, though.
As for Excel-editable. Well, convert2da can *write* CSV files, which are easy to open in Excel. It can't read them, though. So it can only do 2DA->CSV, not CSV->2DA. That's something I might want to add, yes.