Specifically, old passwords were digested in weird ways before being hashed. Notably, account passwords were digested with usernames, so your password stops working if your username is chagned. Not the end of the world, but silly.
Mark all existing hashes as "v1", and automatically upgrade then when they're used or changed. Some day, far in the future, we could stop supporting these legacy digests and delete the code and passwords and just issue upgrade advice ("Passwords which haven't been used in more than two years no longer work."). But at least get things on a path toward sane, modern behavior.