The contract you negotiated and signed from abroad is not the contract you are handed when you land. A new 'in person' version appears with worse pay, longer hours or new duties. You are already in the country, with a visa tied to that employer.
Insist the signed contract is the operative one and any amendments need written agreement. Photograph and keep every version, and note that verbal promises are not a contract.
Poster had already moved to China on a Z visa. They were given a list of things to 'discuss next week' while already doing the work.