On a Cyclone III FPGA, I would like to re-generate the source from a given bitstream. Have you a tool to do this or help to assist me please

We have a customer who has an old version of source code for the above FPGA and an updated bitstream. We need to generate the delta source code so that development progress can continue with the product. Is there any tool or assistance available to enable a bitstream to be reverse engineered to achieve this? We do have an earlier source for it for reference