If you upgrade to Dropbox Plus or Professional, the new space will be added to your existing Dropbox storage space. If you subscribe to Dropbox Plus, your Dropbox quota will increase by 1 TB (1,000 GB). If you subscribe to Dropbox Professional, your Dropbox quota will increase by 2 TB (2,000 GB).
This is in addition to any existing storage already credited to your account. For example, if you already have 50 GB of storage space, upgrading to Plus would result in your quota increasing to 1.05 TB (1,050 GB) during the subscription period.