The argument that you were offered "new" work is valid. The thing is that a refusal of work has many more loopholes to obtain benefits. To further improve your chances of getting UI, say that you were "fired/discharged" when you applied because it's true. Your contract ended. Then on your first claim form, you check the "did you refuse any work" box. That way you are in no way committing fraud.
CBG is so right about not working under the new terms. In your case, you had a chance to think about it, you'll have to sign a new agreement, and once you do that, you can't go to UI and say, "I changed my mind. This commission only structure was too much of a cut in pay." The UI system doesn't protect you when you've negotiated the agreement, but it might be there when the employer tries to make you take something you don't want.
A contract ending is not a firing.
You can bet that if an employer receives a notice of an unemployment claim for someone who was offered and refused a contract, and that person is saying they did not refuse any work, the employer is going to tell the unemployment office about it loudly and in no uncertain terms. I'm not going so far as to call it fraud but there's no way that's going to go unnoticed.
Not in the strickest sense, but I lived that nightmare of thinking I quit, when in the UI world it was a firing and a refusal of work. Let the employer protest on that basis, but if I had to do it over again, I wouldn't have written a letter of resignation, I would have just said, "I'm refusing your offer of continued employment in the poor substitute of a job you're offering." I would have submitted my UI paperwork as a discharge and a refusal of work, and had I done that, it might have spared me waiting 363 days to get my first UI check.
And you might not.