1. VAT return already finalised
Once you have completed one of your VAT returns for the year, you can no longer change the VAT accounting method for the year in question.
For example: In the fiscal year 2016, you complete your VAT return for the first quarter on April 15, 2016. From that point on, you will no longer be able to change the accounting method for the year 2016.
As soon as you reach the next fiscal year (2017), you will be able to change your VAT accounting method, but the change will only be reflected in the annual accounts for 2017 and subsequent years.
2. An opted VAT rate has been applied
If the function to change from the “effective” tax method to the “balance control” method is blocked, this is because an opted rate of VAT has been applied to a booking. As opted VAT rates are not permitted by law when using the balance control method, the change is not possible. However, you can still change the method if you replace the opted rates with other rates or delete the bookings.