You can only transfer from the prior year into the current year from your computer into the Online version. So you can transfer a .tax2017 file into 2018 before you started 2018. Next year you can transfer the .tax2018 file into 2019. You can only transfer (upload) the prior year into the next year at the begging before you start.
If you use the same account next year it will automatically transfer the 2018 online return. There's nothing you can do about getting 2017 into 2018 now.
