In most countries tax rates vary. For example in the UK there is a "special" 20 percent VAT rate which is set to expire on December 31, 2013 when the old VAT rate of 17 percent will hopefully rematerialize. We can plan forward for this.
Consult with a tax lawyer/accountant.
Get the dates and rates of tax. Then we will implement them using the examples mentioned in the book.
Create two tax rules using the Configuring VAT on the entire purchase (including or excluding VAT on delivery costs) recipe if necessary. One at the current rate and the other at the new future rate. You may have already implemented one of them previously.