I haven´t completely understand your question. You want to use a different lifecycle map for existing items?
I don´t understand right now why you cannot edit the classification property. Do you have classification specific lifecycles?
You can reset the used lifecycle map by an admin action. But this will reset the complete lifecycle.
(Translation is something like "set standard lifecycle". Try it in a test database first).
If you already use different states for you items, you then have to re-promote the items after using the action. If you are open minded for a more dangerous but efficient approach: You can also use SQL to update the "state" and "current_state" property that are used for the LifeCycle. It maybe even eliminates the step to reset the lifecycle with the admin button, but I never tested this one. You basically need to know the id´s and names of the target states of your new lifecycles and directly modify them in the database. But try definitely in a test database first.
