If it is REALLY important to do this, your Adagio consultant could walk you through unposting all the batches that you have posted in the new year, moving those batches to the archive, restoring your backup you made before opening the new year, posting the entries to the prior year and then stitching in the archive batch and restoring your work. It's fiddley and complicated, but doable. You'd want someone experienced doing this for you.
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Andrew Bates