sim upgrade reverse

[GDV1]

I noticed I was paying £24 for my sim plan so decided to downgrade as I wasn’t using my monthly data allowance. I downgraded to £16 a month for less data but when I go to see my new bill I'm actually now paying more overall? 
My next bill hasn't come out yet and I was hoping to just reverse what I've done. Does anyone know if this is possible or why my bill is now costing me more?

[caesarome]

It might be part of the bill you are paying for the £24 sim and the other part for the £16 one, if this is the case then it should correct itself on a future bill.