Contract change

Hi. I would like to ask a question regarding to my contract at my first job. I was advised but not sure that i will get minimum to my province salary wage after the end of my training to the company but after the training they give me probation of 6 months with manila rate which rather give me hope and thought they value me but after 4 days of working (having my uniform, ID and other stuff) they want me to change the contract that already signed to another contract that will give me a minimum wage of my provincial rate. In this case I lost my interest and don't like to work for the company anymore. Do they have the right to force me sign the contract? Thank you. I need professional advice.