There are big changes to the code in Adobe RoboHelp 2019
as it is HTML5 and CSS3 based. Projects created in earlier versions will
not be truly upgraded in the normal sense, instead your old project is
left untouched and a copy is made on the fly. It is that copy that is
upgraded. That means that you retain your project in its old format by
way of a backup.
You are nonetheless recommended
to retain a backup copy in either source control and/or a zipped copy.
A zipped copy prevents you opening it accidentally and working on it before
you realise it was the backup. Also you always have a clean backup that
can be used to create a new clean copy time after time. Prefixing the
zip file with the date is good practice. YYMMDD_MyProject.zip.
When you are ready to convert your project created in any earlier version of RoboHelp, then on the Starter Page click Upgrade Project or Click File > Upgrade RoboHelp Classic Project and browse to the main folder of your existing project.
Click Open to display the New Project dialog.

Complete as required and click Create.
When you upgrade, all the image,
media and CSS files will be moved from their existing locations into the
"assets" folder you see below. The folder structure of your
project will be retained for folders containing such files.
For example, if you had all your images in a folder named Images and your
media files in a folder named Media, in Adobe RoboHelp 2019
you will see Assets > Images and Assets > Media. Wherever they are
pre-upgrade, the structure will be mirrored within Assets.
In Adobe RoboHelp 2019
your project will look like this, "project_files" being the
name of the folder that was created in the earlier version.

You can, of course, change your
folder and file names in Adobe RoboHelp 2019
but there's something new. Change the folder name in Windows Explorer
and it will be immediately reflected here. No longer does changing folder
or file names in File Explorer break RoboHelp. If you change the name
in Windows Explorer, note that as soon as you put focus back on RoboHelp,
the change will be shown.