By default propositions are unchangeable by legislature. 7/8ths is a very high bar but most propositions give the legislature no ability to modify it. Further propositions could modify or remove it by a simple majority.

For better or worse, this is the system working as intended. The point of the proposition system is to let the people pass laws the legislature refuses to. It would be pointless if the legislature could overturn or gut propositions with a simple majority vote.