You don't have to create different content or change it because of the change. What you need to do is generate a different output.
WebHelp - This has been used for years and RoboHelp provides skins in the Classic version.
Responsive Help - Here the layout of the content changes to suit the device being used to view the help.In 2019 New UI you use one of the responsive skins but deselect the Responsive option. That stops the layout changing.
Frameless Help - This works in the same way as responsive help but the page is not constructed using frames. Think of a frame in much the same way as a table with cells that hold different areas of what the user sees. The toolbar, the TOC and the topics. Instead of frames, the code sets how the help is set out. It is probably where most effort will be going with help authoring tools.
If you are moving to the New UI version of 2019, see the RoboHelp Reimagined sample project that explains the different layout in more detail.
You have tagged your post RoboHelp Server. That is a separate product and you don't need it to be able to generate any of the above outputs. Behind the scenes it gives you more information about user interaction and so on.