Thanks for the advice not to try to amend the return while it is being processed.
There is tax due, and we had printed out a payment voucher, planning to mail in a check, but the amount (we believe) is now incorrect. It seems like a bad idea to send in the voucher with a check that doesn't match; it would be a nuisance to re-do the entire return just to get a 1040-V with the correct amount. I just found I could download a blank 1040-V from irs.gov and fill in the new amount.
