“The internal string representation has been converted to UTF-8, making strings fully UNICODE capable.”
CHICKEN 6.0 makes strings Unicode by switching the internal representation to UTF-8. It also breaks blobs in favor of R7RS bytevectors, changes process procedures to return objects instead of PIDs, and moves a pile of procedures between modules. Anything written against 5.x needs real work to port. That is the right call for a Scheme that plans to still be here in a decade, and it is still a migration nobody is going to enjoy.