When the existing plans were last upgraded I noticed that I gained the extra quota and retained the bonus quota that I had accrued.
However, the new system of gradually reducing the quota on plans (for new users) does complicate matters somewhat. My guess would be that you would receive whatever was the current quota for the plan you are upgrading to, plus any bonus quota that you had accrued, but that is only a guess. As Wholly said, contact support for the definitive answer.
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