An employee's 'status' as full time or part time doesn't change the rates that must be withheld, or the employer's FICA contributiuons. The change is in the actual hours worked.

If an employee works fewer hours, the amount withheld will be less, and the employer's FICA will be reduced accordingly. The amounts will change, but the rates will remain the same. (A reduction in hours worked may allow the employee to have less withheld, depending on how you determine the annual earnings. Talk to your CPA or a tax attorney for more info.)

Company outlay for various benefits may be reduced (if allowable) by a change in status, but that depends on company policy and local/state regulation.

Hope this helps.

Source: http://www.abusinessmentor.com