It's easier to state which changes do not require a re-index run, actually:
new indexes/fields based on currently empty data, e.g. a new property in an entity that is currently empty for every entity, or a new index mapping for an entity that hasn't any database row yet.
feature reduction, e.g. making a previously stored field not stored, or removing a field. The data will stay in the index, we just won't use it nor update it anymore.