Parish by-laws can be amended, if the parish was foolish enough to put in such a provision in the first place. There are a couple of legal arguments that can be made as to diocesan bylaws, the obvious one being that if the parish has withdrawn, the diocesan bylaws wouldn't apply.
If the parish is in a 'neutral principles of law' state, the court should merely look to the recorded documents to determine ownership of the real estate. In other states, such as Virginia, statutes govern what happens in a split. And finally, there are states that defer to the church hierarchy.
That’s what triggers the problems with the diocese. The idea was that before a parish withdraws it places heavy financing on the property to discourage the diocese from taking it. In most states there are express, implied and constructive trust questions to deal with, not only the recorded title acts. With the Dennis Canon, it just makes it easy for a court to say that a parish has acquiesced in the express trust since it did not protest or withdraw back in 1979 when it went into effect.