Will I always be paid according to my license level?

Not necessarily. It depends on the requirements for the according contest.
If, for example, you are an A judge but the contest only requires B or C judges, you can earn only the amount for the highes license required.

With regard to the payment, the judge with the most experience earns the higher money.
If, for example there are two A judges at a contest that only requires one B judge and two C judges, the A judge with the most experience gets the B judge salary, the other A judge only the C salary.