Goodybags last for one month and once active, cannot be altered, cancelled or supplemented. Make sure you buy one with enough minutes, texts and data to last you a month. It's cheaper to pay a couple of quid more for a bigger bag than you think you'll need than to run out and have to pay from your PAYG credit.
Buying another goodybag whilst you have one active will queue it to start when the current one expires. It will NOT add to or take over from the current one.
The only choice you have now is to use PAYG credit, which will cost you the reduced amount of 2p per MB till the goodybag expires.
Hope that helps