No you don't need to install the 2018 disc first. All you need is to have the data file ending in.tax2018 on your computer to transfer into 2019.
If you used the Desktop program last year here’s How to transfer into the Online version, expand the blue link if you used the CD/download last year........
https://ttlc.intuit.com/community/entering-importing/help/how-do-i-transfer-last-year-s-return-into-...
If you already started an online return you will have to clear it and start over to transfer. How to clear and start over
https://ttlc.intuit.com/community/using-turbotax/help/how-do-i-clear-and-start-over-in-turbotax-onli...
If you used the Desktop program before you may not like the online version. It has less features and no Forms Mode. You can only do 1 return. Your return is only saved online so be sure to save both the .tax2019 data file and the pdf file to your computer when you are done. In case you need to amend in the future or if you need a copy to get a loan or something. You might not be able to access your online return when you need a copy.
