Equal pay dispute or not?

Recently my company proposed a change to contract.

Current contract for drivers, 10 hours per day, mon to fri between 6am to 4pm, overtime at basic rate after 50 worked.

New contract for drivers, 10 hours per day, mon to fri between 4am and 7pm (as required by company), overtime at time and half after 50 hours worked.

if some drivers accepted change whilst others did not and stayed on old contract are they being discriminated on grounds of equal pay, for broadly similar jobs?