I don't see why you would think you should uninstall KLAX first, if there was such need, we would said it clearly.
Just install KLAS using the current installer and it will work OR, if you want to use your old installer, either reinstall KLAX after KLAS OR download and install the Stand-Alone Addon Manager for FSX after KLAS.+
The issue you had with KSFO didn't had anything to do with this, it was just a bug that we found in the Addon Manager that we could only find after KSFO, which was immediately fixed and not an issue anymore.
