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As far as I am aware, it is impossible to have two different workflows associated to an item type. The Admin UI indicates that you change the workflow for the whole item type and not just one item.
As far as switching a type impacts the data, at least for EWM, but likely also for ETM, if the new workflow type has all the workflow states with the same identifiers/names, the value should stay as it is. If there is no matching workflow state, the state is automatically changed to "New" the initial state.
The main message here would be, you can not have multiple workflows associated to items and it is also impossible to switch the workflow for individual items. What you could do, is to customize the workflow.