I assume your reference to "voucher" and "vouchers" refers to Form 1040-ES.
"how do I handle the difference I've already paid on the first voucher,
and the higher amount stated on the new first voucher? Do I even need to
pay this difference at all?"
IF you selected the option to have the Forms 1040-ESs generated based on 100%/110% of your 2018 tax liability AND you expect that your 2019 tax liability to be considerably higher than than 2018's tax liability, THEN I'd say "yes, pay the higher amount shown on the new Form 1040-ESs." As far as the "shortfall" between the old estimated tax amount and the new estimated tax amount for the first quarter payment goes I'd say pay the difference right now, (download a Form 1040-ES for the first quarter here: https://www.irs.gov/pub/irs-pdf/f1040es.pdf ). Since underpayment penalties are calculated on a quarter by quarter basis this catch up payment should help reduce any penalty.
Of course if you don't expect your 2019 tax liability to be materially higher than 2018's tax liability, you could simply continue to use the old voucher amounts.
Questions of this sort - how much to pay in estimated taxes - are just about impossible to answer with any sort of precision since the underpayment penalty, if any, really depends entirely on your personal situation and your own estimates of your 2019 tax liability, something no one here really knows about.
If you went through the W-4 and estimated taxes interview previously, entering 2019 estimates of income, deduction, etc., you might want to go back to that interview again now that you have, maybe, a better idea about those amounts and change your original estimates and see what happens.
Tom Young
