Hello Peter,
PCM2900C was created to meet Microsoft Windows Logo for compatibility with Windows 7. Main difference is changed volume characteristics. There are also changed USB descriptors. It is compliant with USB 2.0, but communication is still full-speed 12Mbps. You can directly change PCM2900C on the board instead version A or B and only difference what you will see is another characteristics of software volume control and line-in instead microphone.
If you want to have microsoft logo on your product, you must use version C of this chip.
Details from TI about differences between versions of this chip are available here: http://www.ti.com/lit/an/sbfa020/sbfa020.pdf
Pavel