2019 amendment program has been available for some time now.
https://turbotax.intuit.com/personal-taxes/cd-download/amend
If you fail to update the program then any bugs they've addressed will not be fixed. As for why go from Online to Desktop: Because you can mess around all you want and then delete. It works great (well as long as TT can get its act together with its 2019 desktop/amend software).
