[rt-users] "Field X changed" Custom Condition?

[Jeff Blaine]

I need to write a custom Condition for a scrip that
equates to 'If Custom Field X's value changed'

Is there an API call for doing that, or do I need to
code the logic to retrieve the old + new values and
compare them?  I can't seem to find a single call for it,
but I wanted to check.

RT 3.8.7