No, you cannot change the data type after the field contains data.   There are 5 data types:
  • Character
  • Date
  • Decimal 
  • Integer
  • Logical.
Each has its own way of being stored in the database.  

While you can't change the data type, you can possibly change the way a field is displayed.   For instance, a character field can change from a Fill-In to an Editor fairly easily.   We recommend that you test your data type choices and View-As choices ahead of time in a practice environment.