If your data is already available as a target value, you don't need to calculate the respective delta for the Valsight import.
Download the excel sheet of your particular line item and change the header from "Change" to "Target".
Afterwards your target values are imported and changes to your existing base line calculated automatically.
Missing rows from your upload which are present in the base line will result in 0 as final (target) value, i.e. your absolute change will be the negative value of your original value.
Be aware that a target upload is only supported for absolute values.