I'm not aware of an upgrade option from Final Cut Express to Pro Studio 2. The only upgrade that I knew of was the Crossgrade promotion which expired in March.
From the small print on the Apple Online Store:
"* The upgrade version requires a valid serial number from a commercial version of Final Cut Studio, Production Suite, or Final Cut Pro 1, 2, 3, 4, or 5. Academic and not-for-resale versions are not eligible for this upgrade."