Normally LNG field have fixed customer....china will have difficulty getting alternative....

Not impossible but china may need to buy at higher price in spot market.

Dpending on the expiry of the contracts, most LNG contracts are very long term. By the expiry of the contracts, the projects would have been paid back in full.
Aus can then sell at lower prices to anyone they want.

Malaysia stand to benefit, we already supplied large quantity of LNG to China. I think recently I read article, China also trying to get LNG from the -STANS nation and relationship with Saudis and Iranians.

They already trying to diversify many products from Australia and the funny things, some of those product they got it from the 5 eyes, like Barley, stop taking from Australia, take from US, dairy, stop taking from Australia, take it from New Zealand.

Except things like Coal, LNG still dependent and paying higher price. But I think eventually it will shift as well.

The reason is simple, it's geopolitic. Australia is the staunches US ally in APAC more so then Japan. China cannot hit US directly and China can surely whack Australia and since it can, it will do so.