C51 v6 introduced some major changes. The documentation includes a list of changes - you will have to read it to see if any of them are significant to your project.

Note that compiler changes can affect both the source code, the code generation and the command-line options...

Are you sure that it all builds with no warning or error messages?

It could just be that your code was always dodgy, and you just happened to get away with it before; you were just lucky.

What do you mean by, "the code doesnot run?"
What does it do?

Have you tried it in the simulator, to see where it falls over?