If I buy a ECE r 44 04 product will I have to change to i-Size within the usage period?

No, you can use ECE r44 04 seats as long as the National legislation will allow this, which will be at least till 2017 and depending on National legislation of each country. So for many years ECE r44 04 child safety seat and i-size child safety seat will be both available and are allowed to be used (at least till 2017).