Changing TAX to Including from Excluding

We currently sell all products as + VAT to clients with the EU, and we are looking to change this to show the product as VAT inclusive from now on.

However, we are needing a way to update the prices of all products in the database by 20%, to exclude any client that is located in a country located in one of the TAX Rules.

Clearly this will need to be done at database level, but we are looking to see if anyone can help with an SQL statement that could achieve this.

Thanks for any help.