If the 2019 Form 1099-R includes code P in box 7, the earnings shown in box 2a of that form are reportable on the 2018 tax return and the 2018 tax return will need to be amended to include the 2019 Form 1099-R. If the 2019 Form 1099-R instead includes code 8 in box 7, the earnings will be reportable on the 2019 tax return.
Since there is no immediate need to amend, I would wait until you receive the 2019 Form 1099-R near the end of January 2020 and then amend 2018 if the form includes code P in box 7.
