highlight-changes-global-changes-existing-buffers

highlight-changes-global-changes-existing-buffers is a variable defined in `hilit-chg.el'.
Its value is

nil

Documentation:
If non-nil, toggling global Highlight Changes mode affects existing buffers.
Normally, `global-highlight-changes' affects only new buffers (to be
created). However, if `highlight-changes-global-changes-existing-buffers'
is non-nil, then turning on `global-highlight-changes' will turn on
Highlight Changes mode in suitable buffers, and turning the mode off will
remove it from existing buffers.

You can customize this variable.