Oi Pigboat - what you got against Oktas? What they ever done to you?!
BECMG is certainly ICAO standard (used it all my meteorological life when I had a meteorological life, now I've just got a life...). It means that the weather is forecast to change in the time period mentioned (0700 - 0900, or 0709) from the previous conditions to the new weather conditions - so its transitional and the weather could be one, or the other, or somewhere in between, and it'll be a permanent change. Like forecasting a front is going to clear, or in this case, the fog is going to lift for the rest of the day.
Could also use FM0900 (From 09:00), where the forecast change takes place from 09:00.
Cant find any online official references (ICAO / Jeppesen), but a simple online search on TAF Decode will bring it all up for you.
Now thats either going to confuse you more, or help...