Yes you will lose the current double data bonus if you change your tariff. However, there is currently a double data offer on till the end of the month so you can get it again. Currently, the same as you've got now is £12 - for 2GB doubled to 4GB (with 2GB of rollover possible). Bear in mind, though: Service numbers (084... and 087...) are no longer included and are expensive to call (if that matters to you). Currently 20GB is £15 a month.