There is nothing to enforce about a hardfork. This proposal will produce a mutually incompatible protocal, so unpatched clients would reject it as invalid, and

There is nothing to enforce about a hardfork. This proposal will produce a mutually incompatible protocal, so unpatched clients would reject it as invalid, and patched clients would reject the unpatched clients as invalid. This is in contrast to the accidental fork we saw last year (or was it longer?), where the new client would accept as valid the old clients chain. Practically, this could be implemented as blocks after block X must follow the new rule, blocks X and earlier must follow the old rule. Where X is sufficiently far in the future to allow for people to update.