If you want to pay your employees using different pay rates within the same pay period you should define either one of your twenty user definable pay rates for the old rate, or setup a taxable allowance representing the old pay rate.

If using a taxable allowance you are best off to set the calculation method to Fixed Unit Value, then set the unit value to $1.000. Then enter the gross for the old pay rate as the number of units of the allowance.

Ensure you have filed your current payroll prior to changing the employee pay rates, otherwise the unfiled pay will be recalculated for each employee at the new rate.