Hello,
I tried to install the mod with an existing save, but it does not seem that costs have changed in accordance to previously researched technology.
Am I doing something wrong or does the mod only work for new saves?

Well... I might not be very actively monitoring these messages, so forgive me for such a late reply. The research cost does not change until you have researched at least one new technology with the mod enabled. The mod calculates and adjusts the research cost after every finished research, so if you add it into an existing save you won't see the difference immediately.