org-log-reschedule

org-log-reschedule is a variable defined in `org.el'.
Its value is

nil

Documentation:
Information to record when the scheduling date of a tasks is modified.

Possible values are:

nil Don't add anything, just change the date
time Add a time stamp to the task
note Prompt for a note and add it with template `org-log-note-headings'

This option can also be set with on a per-file-basis with

#+STARTUP: nologreschedule
#+STARTUP: logreschedule
#+STARTUP: lognotereschedule

You can customize this variable.