As I understand it - the tax code is cumulative - so they divide the allowance between the 12 months - you get 1/12th each month and as long as you were not on month 1 coding it accumualtes so that at the end of June which is month 3 of the tax year you will have 3/12th of the allowance used -when the new allowance is implemented at the end of the month you will get 3/12 of the new allowance and the tax ytd to date will automatically adjust and you will see the money in your payslip this month. I hope that makes sense