What happens to the price when you change the setting?
For example: If I had it set to ‘Prices Entered With Tax’ and I add a product at £100 – that would mean the price would be £83.33 + £16.67 tax (20%)
If I changed the tax setting to Prices Entered Without Tax would the existing prices of the products be changed to the before tax value?
Or, would the price stay at £100 – but have £20 tax added in the shop/checkout?
If the price isn’t changed then you’d need to edit it to make sure the correct price is used in the shop – easiest way would be to export the products to a CSV, change the prices in Excel and re-import!