With respect to the changes for 2018, since your Form 8606 must include Part II, I would include 2018 Form 1040X even if the taxable amount of the Roth conversion was zero (which only seems possible if you contributed for 2017 and 2018 simultaneously and immediately converted to Roth, otherwise I would expect as least a small amount of the Roth conversion to be taxable). This will allow you to explicitly show that there is no change to your taxable income or tax liability.
