I am on an existing payment plan with the IRS. Will filing an extension this year cancel any existing payment agreement?

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No, filing an extension for your 2018 return will not affect the payment plan you are on.   You still have to keep on making your regularly scheduled payments per the installment plan.  An extension for your tax return does not give you extra time to pay.  It gives you extra time to get your documents together to file the return.  Any tax due on a 2018 return is still supposed to be paid by the filing deadline of April 15, 2019.  If you are going to owe again for 2018 you will need to work out the adjustments to the installment plan with the IRS.