Hi Fred,
I guess you want to do this without loosing the current price of your products right?
Some people already did it in this thread
did it in this thread
and it seems the best way to do this is to execute an SQL request in phpmyadmin.
In your case, the request should looks like this:
Code:
UPDATE #__hikashop_price SET price_value = price_value*0.21/0.19;
Then, just change your tax rate in hikashop and it should works fine