I think the message is quite clear: is telling you have an outdated version of a scenery, and is suggesting to download and install the current version.
I'm not sure what do you mean with "reload from scratch again", you only need to download and install the current installer, shouldn't take too long. You can uninstall or not the previous version, it won't make any difference.
If we release a product that *requires* to uninstall the previous version, we'll make sure the installer will not proceed if you don't do that, like the KORD installer.
In your case, it's probably safer to install PHNL after Zurich.