Sorry to hear about that, what a pain.
Based on some advice I once got from Oshyan, I always duplicate the Terragen folder immediately after installing a new version. The new folder is named with the version number and then I make a shortcut from the tgicli.exe file to the desktop. That way new installations won't overwrite existing ones, and I have a complete set of previous installations in the Planetside Software folder.
Too late to help this time, I know.
But maybe for future reference.
He gave me this advice a couple of years ago, so I'm not sure if/when the installation method changed from the way you describe it.