When you change the trigger for a workflow it breaks all of the dynamic fields in the workflow which is fine. The concern is that if workflow notifications are activated and they also use dynamic fields then this is not immediately apparent to the end user. In addition the WF builder wont let you save the updated WF until these fields are corrected.
As a secondary suggestions I'd recommend some kind of pop up when you are looking to change the trigger for the workflow to notify the user that the dynamic fields are going to break.
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