Based on recent Geekbench scores, it would appear that the change was made last month. A Surface Pro 2 owner, who had a problem with his slate following a firmware update, exchanged his tablet for a new one and noticed the change. Besides the faster speed, the newer chip also includes Intel Trusted Execution Technology, which offers enhanced software security.
The reason behind Microsoft's decision to replace the original processor on the Microsoft Surface Pro 2 is unknown, but the updated slates are just hitting stores right about now.
source: TheVerge via Pocketlint