How does this upgrade to 2019 work?

Have a straightforward question. I have the X3 version and I'm seeing an ad on my screen telling me I can upgrade to the new 2019 version for $99.00. My question is if I chose to do that and the time comes for me to download the new setup file, what am I downloading?

I mean is the setup file the entire 2019 version that I install standalone or would I first have to install the older X3 version and then on top of that install the 2019 upgrade file?