The former "Left F/L 310" is far better English than "Leaving F/L 310" and means what is says- which is you have departed F/L ??. "Leaving" is a nebulous term loved by ICAO and can be taken to mean "I haven't actually vacated F/L310 yet but hang in there old chap I will be leaving F/L 310 asap.
I haven't a clue of the rationale for changing what was a perfectly understandable Australian ATC phraseology all those years ago. Rather than bend to a nebulous ICAO RT recommendation, it is time to stand up for commonsense English grammar and if necessary file a Difference to an ICAO Recommendation.