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They can change the way they pay you, but it would have to be part of the agreement. If you and the employer cannot come to an agreement the employer can make their "last best offer" and it is then up to the union to either accept the proposal or the employer declares an impasse and can implement the last best offer and then force the union to accept or seek binding arbitration against the employer.
So they can legally make their final or last best offer, if the union rejects, the employer can declare an impasse and implement the last or final best offer and force the union to accept it or seek binding arbitration on the issue. Unfortunately, the way these matters have been going lately, if the compensation plan is reasonable, arbitrators have not been overly friendly to the unions on this type of issue, especially where there has been essentially an agreement on most of the other issues.
The alternative is for the union to regroup and decide what other issues they could possibly give up to try to bargain to get the compensation package they want for the drivers, which has come to be known over the last few years as "concession bargaining" which means the union has to give things up to get other things, even though the union really does not want to do so.
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