My taxes were submitted and processed. I've selected to pay estimated tax for next year as well. Can I change the actual amount of the voucher at the time of payment?

Yes, you can change the amount of payment on the voucher.  As an IRS  employee once told me, "We care about the check, not what's on the voucher."

Yes, if you need to you can. The best thing to do is to print off new vouchers and write in the amount that you are paying. You can get them here: https://www.irs.gov/pub/irs-pdf/f1040es.pdf