Simply installing again should trigger a repair, which should configure AutoCAD 2015 in that case. Can you repeat the test, but see if a manual repair works as expected? Also, please test the latest 8.0.0.7 in case there was a problem with the installer only in 8.0.0.6.
The user did exactly what you said and it solved the problem.
However, when he tried to install several times my application, the problem has never been solved.
So I think that the InnoSetup project is set to not reinstall OpenDCL.Runtime.8.0.0.6.msi, when it is already installed in the system.
Unfortunately, now I can not do the test, and I have to trust what he says my client.
Next week I will be more precise.