It is probably legal for them to request a change in terms of the contract, but you are under no obligation to sign the new one. They do have the leverage of outright dismissing you. Depending on your work history over the past 15 months, that leverage becomes nearly null if you're financially eligible for unemployment. Basically of you've worked the entire past year (for any combination of employers, and made >$600 from this one). The actual rules are less ballparked than that, but check the FAQ on the DoL website to make sure.

Changing the contract benefits only the employer in this case. They want you to work an extra eight hours weekly for the same paycheck. There's honestly zero incentive for you to sign this new contract.

I would start looking for a new job regardless, they seem scummy.