This upgrade started with usual goal - to upgrade the clock (or recovered clock) inside DCX2496. I started the backengineering and found out that the CS8420 receiver chip is used in a hardware configuration and in such mode that the clock is not recovered from spdif stream but instead internal oscillator of 24.576 MHz is used. Also I found that the actual clock on DAC chip is exactly half of the main clock.
Luckily thanks to Davey at DCX2496 Yahoo group the schematic of the unit was posted. This made the rest of the upgrade a bit easier.