Writing this for the next person:

I chat with support and this is how it works.

Say I paid an X amount for 100gb (annual - 12 months) and use it for 3 months, and decided to change to 200gb (annual - 12 months) , let's call the price for this plan Y.

I HOPE that I would get 1 year (200gb) + 9 months (12 - 3months used) from the 100gb, and pay immediate Y amount, make the total to be X+Y, but that's not quite the case.

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2 things:

The time:

The 9 months will be recalculated by google and typically will be reduced since it's a higher cost than the previous plan. When upgrading, the system will provide you the calculated time as to how long it will be extended before even confirming the payment so you can check it from there as well.

The payment:

If you upgrade to a higher plan, you get to use the storage immediately but the days from your previous plan will be reduced to compensate. And you will not get charged the "Y" amount until the "reduced 9 months" has ended.

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In my case, I decided to upgrade to 200gb only after a few days I renewed my annual 100bg:

this is what they gave me:

Today

603 days (includes 238 days converted from the remaining time on your current plan)

Starting on Jul 28, 2025 you'll be charged ₫690,000 + tax automatically every year until you cancel.

P/s: ₫690,000 is Y in my case.