It does not matter whether you e-filed or mailed the return. You can transfer your 2019 data to the new 2020 return at the very beginning--BEFORE you enter any other data in to 2020. If you amended, that is what will transfer in.
How to transfer last year’s return? https://ttlc.intuit.com/questions/1900883
When it's time to efile 2020 you will need to enter the AGI from your original 2019 return for verification (not the amended return). If that doesn't work then enter zero for the AGI.