If the user tries to change modes, a warning dialog is displayed:

     Changing Modes Clears Calculation

     When you change modes, the current calculation 
     will be cleared, and the base will be reset to decimal.

   There is a checkbox that can be toggled that says:

     Do not warn me again

   If the user presses the Cancel button, the change mode operation is
   cancelled. If the user presses the Okay button, the mode is changed.
   This has the following side-effects:

   * Clears the display
   * Sets the base to decimal
   * Sets the numeric display to fixed
   * Sets the accuracy to nine places after the numeric point.
   * Clears the display of the thousands separator
   * Clears the display of trailing zeroes after the numeric point.

   and dismisses the register window if the new mode is Basic.

The documentation needs to be updated to reflect this.

(Originally filed as bug #153854).