Even changes of major properties do not always warrant “resetting the start_date”, unless the change is so dramatic that the feature is now perceived as new instead of as an upgraded continuation of the old feature.
It helps to be specific w.r.t. what OSM element start_date is applied to. For example, if a railway station gets a new station building, you can set the new date as start_date for the building element but keep the old date as start_date for the station element.