How do I change voucher dates on admin vouchers that have already been paid?

There is no admin voucher date fix, however there are a couple of ways to handle it.

One way is to create journals to move the expense and payable from the wrong period to the correct period, but the BEST way is to void the check and create a debit memo to remove the voucher, and re-enter the voucher with the correct information.  Then apply a hand check or payment item to the voucher - using the original check number. You will get a warning about duplicate invoice numbers and duplicate check numbers, but you can ignore them.

More detailed steps are:

1. Find the check number associated with the voucher in the Payments tab in Accounts Payable.
2. Double click the check and then click the Void Payment button in the lower right hand corner.
3. You might get warning that this check has cleared the bank - click ok to continue since you will be putting the check back into the system as a payment.
4. Choose appropriate dates for the check void and do click the Create Debit box.
5. Select a Void Reason or type it in if desired and click OK.
6. Process the check void.
7. Navigate to the Item Entry tab and Edit and Post the Debit memo. You do have an opportunity to change the dates if you need to.
8. After processing this all through, the voucher will need to be re-entered with the correct information.  Since this is an Administrative Voucher, you can enter the check information before posting.  This will save you a few steps.  To do this, after creating the voucher and the line items, click on the Hand Check radio button and enter the check number, date and amount of the check.
9. Edit and Post the Admin Voucher\Payment and you are done.