scientiae: For instance, I always hesitate to install some new modification (correlated with the enjoyment of each game) because I know there is a good chance that the game that I have working just as I like it will fail and I will have to research how I installed it, again, because often there are a number of steps to the installation after the initial Gog offline executable runs.
Mods don't tend to alter the registry, so simply making a copy of the installation folder before applying any mods would leave you the option to easily restore things back to their original state and whenever a game doesn't rely on the registry to tell where its files are located, I would suggest you to leave the original installation folder intact and only mod the copies you created from it.