Late Form 8606 with our without Amending

I realize now that I need to go back and File Form 8606 for 2017 (when I made a nondeductible contribution to an IRA), and also for 2018 (when I made additional nondeductible contributions, and converted it all over to Roth).

 

I have a pretty good handle on how to fill the Form out for each year, but my question is if I need to Amend my return for 2018. I've spent a long time on these forums and the consensus is that it's not necessary for 2017, when I simply made a contribution, but there I've gathered both YES and NO answers for if I need to amend my 2018 return, or simply submit the 8606.

 

Looking for clarity, please!