I have not seen any upgrade path. You might want to put in your Headphone Review there is a new DAC option for newer units.
They have stopped selling DAC192 in the Phonitor X. It also looks like it is a Factory or Dealer installed option
The Phonitor E also now has this new DAC as an option.
This was on SPL Facebook page, it was announced this last August.
[August 31, 2019].
New Product 
DAC768xs
Phonitor x and Phonitor e are now available with the new DAC768xs. The DAC768xs is a small version of the DAC768 and offers almost the same range of functions in the digital domain like the reference model DAC768, which is used in the Phonitor xe and Director Mk2. The DAC768xs is less expensive and smaller that’s why we call it xs.
Funny how they describe as almost the Jr DAC to what in the Phonitor xe and Director Mk2.