As long as it's not in a closed year Sage 50 will let you do it. The new cash receipt will not be marked as cleared on the bank rec. You will need to go back to the bank rec for that period, unclear the deposit the receipt was a part of, save the reconciliation, then go back in and clear the deposit again.

Assuming the original receipt debited cash and credited an income account, then if you use the same income account on the new invoice your financial statements won't change as long as both the invoice and the new cash receipt are posted to the same month as the original transaction.