@gus4113
Your issue is the same if you do not pay the entire 2019 balance due by 4/15 or 3 months later ... when you are on an installment plan it can be null and void if you do not keep up on all tax liabilities in the future ... so the IRS could cancel the payment plan and demand payment in full if they will not allow you to adjust the current plan to include the new taxes owed.
