If you’re using 32-bit XP Pro, then it’s likely you formatted the drives as 2TB NTFS
Since you’d be replacing 2x 1TB drives with 2x 2TB drives in the v2, you’d be going from 2.7 TB to 3.6 TB available physical storage on your Drobo v2.
Since you already have 4 TB of storage provisioned (2x 2TB volumes), you shouldn’t end up with another a drive letter. You’ll just have 900 GB more free, added to your 506 GB, would give you 1.406 TB free between the two drive letters, H and I.
You’ll only end up with a new drive letter when your physical storage exceeds the provisioned storage. That would happen if/when you replace another 1 TB drive with a 1.5 TB or 2 TB drive, because you’d have more than 4 TB of physical storage, so another volume would be created.
Free space is shared among all volumes on the Drobo, so there’s no real amount you should keep “free” on each volume. Just make sure your total data doesn’t exceed your total capacity. Drobo will warn you before that happens.