Well, the message gives specific examples of the kinds of code that might need to be modified: "debuggers, macroexpanders and code-walkers".

I think this is incorrect, the point of Guile-Emacs is to implement support for all Emacs Lisp in Guile. No backwards-incompatibility will be present in whatever Emacs eventually moves to.

One example is that some extensions such as Gnus have contain code written in the current Elisp interpreter's bytecode, which is incompatible with the Guile VM.

From the submitted email:

>Guile-Emacs loads, compiles and executes the programs included in Emacs by default, plus Gnus, Org-Mode, and more.