It is possible to change a Field's name and properties at any time, even if you have already entered data into the Field.
To modify a Field's properties, simply go into Design Mode, open the Structure Pane, and double-click the Field you wish to modify. Be sure to click the Apply button in the Inspector to save the changes.
Note that if you change the Field's Type the data associated with that Field will not be modified until each record is saved after the change. This allows you to switch the Field Type back if needed.
Be aware that there are some limitations. For example, a Subtable Field cannot be changed to any other Field Type, and data from a Picture Field converted to any other Field type does not display. Refer to pages 225-228 in the 4D 2004 Design Reference Manual for more information. You can download the Design Reference from here: