It depends what kind of transaction it was.  What was the transaction type?

If the work order in question is still open, the cleanest thing to do is reverse the original transaction with the 2004 date and reenter it with the correct date.  If the work order is closed and you aren't posting work order transactions to the GL, then you really don't HAVE to do anything.  However if you ever run work order transaction based reports such as JC-E  or JC-M and include a date range, you may be excluding the transaction in question.