My contract states that the company will provide a housing allowance, and it must be adjusted to reflect market conditions to provide suitable accomodation.
Company policy provides the details, and this can be changed, or cancelled by them, it's true. But the onus is on them to continue to provide me with suitable accomodation as per my contract - which cannot be changed for the worse without my consent.
We have data, and an agreed benchmark: The HK Government index. So for the company to decide to try and change our agreed definition of "suitable" will be hard to do.
The only way would be a new contract. But nobody would sign it, surely?