48 v Three mobile

Hi all! I have been with Three for a number of years and recently, am being tempted by the offer of 48. I'm on prepay plan, where every 28 days have to pay €20 to keep the unlimited Internet data. Honestly, don't even use it much as these days I can hardly leave the house...

So thinking of swapping to 48 instead. Good idea or nay? Second problem: I have a lot of credit on Three left, since I haven't updated the plan so topping up doesn't take the €20 and it just built up. If I do go with 48, what happens to the credit? Do I just lose them??

Thanks in advance!