Historical data will be updated to 12:00 timestamp when changing a Date field to Date/Time field. According to the Salesforce documentation, "When you convert a Date field to Date/Time, Salesforce appends "12:00" to existing data in your records." The change will not be instant, but will require some time for processing. The field name will not change unless specified by the app builder. Data loss will not be experienced, but data accuracy might be affected.