Actually, you'd have to change almost all of it. Depending on what you would refer to as the "suspension." No, you won't have to change springs or dampers, but the rear disc brake conversion certainly will require different control and trailing arms. If he just got rotors (for another Civic/Intergra), you're not gonna be able to us his old ones 'cause you most likely have drum brakes in the rear (unless you had a CTR). There aren't any mounts, calipers or lines that will work with a rear disc brake. If you still insist on not changing the rear suspension, maybe you could perform some custom fabrication, but then it'd probably cost more than to just get a rear disc brake set from an Si or 'tegra.