Optional fallback
If an equivalence relationship exists, the original cert(s) are automatically usable as fallback(s) (unless revoked). If the key owner loses the secret key material for their original, they may wish to state explicitly that the original is no longer acceptable as a fallback - but currently this can only be done by breaking equivalence, which will prevent their correspondents from upgrading to the replacement if they do not already have it.
It may be desirable for a key owner to allow upgrading from the original to the replacement, but prevent fallback from the replacement to the original.
(This may also be related to #14)