Solved
Which form do I need if I want to add my taxes due this year to my current Installment Agreement?
You will not need a form. When the IRS receives your return with an amount due that is not paid, they will cancel your installment agreement.
You will then receive a notice of the cancellation in the mail.
At that time, you will want to call them and combine the two amounts into a new installment agreement. It is not a problem, they do it all the time.
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