Google Cloud Storage: what happens to billing after storage class change on an object

I would need to better understand what it happens to billing when I change the storage class of an object.

I have a bucket on Google Cloud Storage; inside the bucket I have an object of 100Gb saved as Nearline. Then after 15 days I will change the class of this object to Standard. As the nearline has a retention period of 30 days, it is not clear how I have to calculate the price. Considering the whole month, do I have to consider a 15 days of storage at nearline + "early deletion" period of 15 days, and at last another 15 days of storage at Standard class price?

Regards, Matteo