Existing Installment plan for 2017, but new one needed for 2018

[BR8]

I have a direct debit installment plan for 2017 federal taxes I owe. I just found out that I will owe money for 2018 that I will need to pay in installments, too. I have been doing some research here in the community, and online. Is my understanding correct?:

The 2018 taxes owed can be added to my installment plan. 

This can be arranged through either:

1. When I submit my 2018 tax returns, the IRS will send me a letter stating that my 2017 installment plan is cancelled (by not paying 2018 taxes in April, I violated the terms of the existing installment agreement). At that point, I can contact the IRS and ask them for a new installment agreement for the total amount owed (ie both years).

Or

2. When I submit my 2018 tax returns, I can complete the form IRS 9465 Installment Agreement Request through turbotax, and include the total amount (the remaining for 2017 + estimated 2018) owed on that form. 

Is one way better than the other? 

[DoninGA]

IRS website - https://www.irs.gov/payments/online-payment-agreement-application

If your payment plan is paid through Direct Debit, you must contact us to make a change.

Call the IRS: 1-800-829-1040 hours 7 AM - 7 PM local time Monday-Friday
When calling the IRS do NOT choose the first option re: "Refund", or it will send you to an automated phone line.
So after first choosing your language, then do NOT choose Option 1 (refund info). Choose option 2 for "personal income tax" instead.
Then press 1 for "form, tax history, or payment".
Then press 3 "for all other questions."
Then press 2 "for all other questions."
- When it asks you to enter your SSN or EIN to access your account information, don't enter anything.
- After it asks twice, you will get another menu.
Then press 2 for personal or individual tax questions.
It should then transfer you to an agent.