No upgrades are available. Also note that Ultra Analog is out of business so in the unlikley event that the chips go bad replacement is difficult. The transport in the Cesium is likewise no longer made. An "orphan" product although Resolution Audio is still in business.
The Quantum was a damn fine DAC that was compared to units costing much more money by Robert Harley ($3,000 versus $6,000+) -- he unfortunately got hung up on Resolution's recommendation that *TWO* DAC's be run in a balanced configuration (only possible with the pricy Cesium transport). You can do this now with any transport if you can find a Chronos (Resolution's anti jitter device).
Anyway, the Quantum on its own was a bit of a giant killer in its day -- considered better than the EAD 9000 and close to the then top of the line Sonic Frontiers. This is one of the most puzzling reviews I've read. He describes the DAC's qualities, favorable compares it to much more costly equipment abd then pans it. Go figure. Anyway, a good DAC for current re-sale prices.
Suggest you call Resolution Audio for a more informed opinion.