Hi Rob,
If .lisp files are actually user programs, then I'm inclined to think it means that Ufasoft have simply re-implemented the CLISP engine (i.e. the binary that runs these ordinary user programs) in C++ rather than C which the original CLISP binary was written in.
No changes are needed for the .lisp user files to run in both the standard CLISP environment written in C and this new CLISP implementation re-written in C++.