Can we change the Rate from 'Pretax' to 'Aftertax' after loading and enrolling the Employees?

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We had loaded the enrollment data from the previous system and now during testing, its found that some of the rates of plans should be 'Aftertax' instead of 'Pretax'. Is it possible to retroactively change and then start working once we integrate with payroll or should disenroll and enroll them again?