If you changed your plan in a middle of a billing cycle, you would only have access to a partial “prorated” data amount of the new plan depending on how many days left in the current billing cycle. In the next billing cycle, you will have access to the whole data bucket. 

You can find detail explanation here https://www.koodomobile.com/en/help/proration

That’s the reason why it’s best to change plans right at the beginning of your new billing cycle.