Why not retrieve the batch into Ledger and post it, letting the transactions for closed periods fail and end up in an error batch? Then move the error batch to a new batch, setting the fiscal period to the current one and posting them. That way the invoices end up in Payables with their correct date and you change the fiscal period in Ledger.
Of course, your Open Payables report will not tie to Ledger, but it probably doesn't at the moment.
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Andrew Bates