A rule change is really safe only if all miners and all clients upgrade to the same version before the change is activated.
bitcoin is not a democracy. ergo, nobody "enforces" anyone.
Self-interest is supposed to convince all clients who care about their coins upgrade -- if the majority of the miners have decided to implement the change in the protocol, if clients are told that there will be a change, and if they are clearly warned that their software will stop working on date X, unless and until they upgrade.