If you are cover with EA, the employer must follow exactly the EA. EA win against offer letter. Extra benefit then is depend the employer.
If you are not cover with EA, then the contract of employment you need to fight for your benefit. Don't be shy to talk with your HR department or boss. (Contract win against employee hand book)
If they change the T&C, they need to prepare a new offer letter to you. If you don't like, then don't sign. But you will lost your job. If you want to stay at the company, then you need to take the offer.