In the switch from "PLT Scheme"/mzscheme to Racket backwards compatibility was almost entirely maintained, you can still use `mzscheme` or `#lang scheme`. Ther

In the switch from "PLT Scheme"/mzscheme to Racket backwards compatibility was almost entirely maintained, you can still use `mzscheme` or `#lang scheme`. There were some substantial backwards-incompatible changes before that (~10-15 years ago). For example, revamping the exception hierarchy, moving to immutable pairs, and re-doing the class system.