From ATC (I don't have paper copies anymore so can't give page number)
1.10.1.4 After any frequency change, pilots must advise the last assigned level and, if not maintaining the assigned level, the level maintaining or last vacated level; eg, “MELBOURNE CENTER (CALL-SIGN) CLEARED FLIGHT LEVEL TWO ONE ZERO, LEAVING FLIGHT LEVEL TWO NINER ZERO”.
NOTE: The “last vacated level” may be omitted by identified aircraft squawking pressure altitude derived level information.
If they want us to say leaving here, why would it be different for any other call?
It's not hard to do it correctly.