C++ is backwards compatible the same way and there was never any need to migrate your code. What happened was that c++11 brought in a major set of new features, changing the best practices of how to use the language, almost to the point of making it a new language. Mainly smart pointers, initializers, type inference, threading and lambdas, and removing tons of other small rocks in the shoe.

So that someone familiar with c++11 or newer opening up a legacy code would maybe not have a hard time understanding it but they would go "wtf is this verbose shit" and be tempted to rewrite it. Vice versa an old schooler would need to relearn these new features when opening a new code base.