If you are not making major changes to the code, then you can try this.
Make a copy of the original dll with a different name. Open the project for the original dll, go to project properties, and under component tab select the option for Binary compatibility. In the file name space below the option define the path and file name of the dll copy.
Now if you compile the dll, it will try to maintain binary compatibility with the original file. Applications using this dll will then be able to recognize it.