This immediately suggests what you want is a gradual migration process from one language through several intermediate languages to your target language.

i.e. instead of a big bang C++ -> Carbon migration, instead you want something like:

C++ -> C+++ -> Carbon-- -> Carbon -> Carbon++ -> Rust

(Basically fake names for more gradual transitional intermediate languages, assuming Google would like to have everything in Rust eventually.)

The key idea of "targeting automated migrations" makes this kind of thing feasible