freezing_rain: In fact it will NOT replace your old Galaxy installation. The default installation folders are two seperate folders and installations. The only thing that might get replaced are your shortcuts on Desktop etc, because they're named the same.
But look at your installation folders and you will find both installations. It won't overwrite your old Galaxy installation.
The default folder for the old installation is "GalaxyClient", while the default folder for the new 2.0 is "GOG Galaxy". You will find both versions still there and can start both (though not at the same time), if you installed 2.0.