You should change the data type of the original Application Date to a string, and then create the calculated field as shown below. You could then replace all old references of the Application Date with the new calculated field.
CASE [Course year]
WHEN 2018 THEN DATEPARSE("yyyy-MM-dd",[Application Date])
WHEN 2019 THEN DATEPARSE("dd/MM/yyyy hh:mm:ss",[Application Date])
Hope this helps.