ldonyo: My Games page has Fallout listed as updated and the installer is for version 2.0.0.14 for Windows. I went to install it over the top of my current install and it claims there is an illegal character in the file path. Since the period is a valid character in a Windows file path I'd like to know why the installer thinks this is invalid. Also, why can't the new installer find a previous install of the game?
GOG's removed the period from all of their file paths in their recent installers, some games just can't handle them. They were installing games to "Program Files/GOGcom" (instead of "/GOG.com") for a while before switching over to "C:/GOG Games". So my hunch is, to keep people from installing those suspect games in folders that do have periods, they just blocked the option in their newer installers. And since it's a code change in the installer's code, it applies to every game using the new installer, even games that don't have the dot problem. Too much of a hassle to do otherwise.
I think that probably answers your question about "finding" the old install as well, but just in case, I'll address it. The issue is really that... GOG's installers don't care to find the previous install, and so will never "find" it. It's not a step the installer actually performs. It's kind of blunt that way. Even if you installed the game and then installed it again in the same place, it wouldn't notice: it would just overwrite the same files with the same files, oblivious.
If I'm reading what you're saying right, your 1.X install of Fallout is in /GOG.com. but the new installer can't use periods. The easy answer is to uninstall the old version, install the new one, and move your saves. An easier answer is to not change at all, because I don't believe the 2.0 installer adds anything but the 2.0 installer itself (GOG's been sifting through the games, silently updating the installers for the past year or so, that's why we didn't see an update until the Mac version was legitimately updated). So you're probably fine without it. Someone correct me if I'm wrong about this being an installer-only update.