A note about unposting GL batches.
In order to be able to successfully unpost GL batches, you must make sure that all of the GL accounts are set up as detail, and not consolidated, type accounts. Changing the type to detail will only allow subsequent batches to be unposted.
Also, you cannot unpost a batch that was posted using the "Post to prior year" function.
If you meet these criteria, Andrew's method should work for you.