Section 125 Cafeteria Plan Correction

Our company has had a qualifying Section 125 Premium Only Plan since 1992.  We just discovered that a couple of payroll items do not appear to have been set up correctly when we implemented QB Enterprise 2 years ago.  We have 4 payroll items associated with the POP.  One for Office Employees.  One for the Shop Employees. One for a Dental plan and one for a Vision plan.  For the first two items, the Company shares this cost with the employee.  For the Dental and Vision plan, the employee pays the full amount but it is sponsored and paid by the company using the employee payroll deductions.  It looks like the 2 payroll items for the Office and Shop Employees was set up as "Premium Only/125" under the Tax Tracking Type and I believe that is ok.  The Payroll items for Dental and Vision were not set up as "Premium Only/125."  They were set up as "None" on the Tax tracking type screen.  The Vision item had the "gross pay" button checked on the Gross vs. net screen and the Dental item had the "net pay" button checked on the Gross vs. net screen.  I believe this may mean that the tax withholdings have been incorrect from the beginning.

First question:  Do we need to change the payroll item tax tracking for Dental and Vision to be "Premium Only/125"?

Second question:  If we do make that change can we simply make the change on the current payroll item or do we need to set up a new one.  If we make the change to the current item, will it make an entry to change all records retroactively?

Third question:  What do I do about the tax withholding errors from the past?  And how would I go about calculating what the amount of the corrections should be?