In principle the question of upgradable DACs seems to me as a bit of a conundrum. With the price of some very fine DACs in the $1K range, it would depend on the affordability of the upgrade plus the shipping involved. If an upgrade were offered which was $100-200 then perhaps even with shiping that route would be wise. However, it would be a very rare company which would offer such economical upgrades; more likely you'd be into it for approx. half the cost of the new unit by the time you were done.
However, if upgrades were in the $300-400 range the question should be asked, what is the holistic advantage of staying with the company upgrading when perhaps an entirely different DAC may offer far superior performance for about $500-600 more? With the sale of the owned DAC and avoidance of the upgrade fee the equation is not so skewed in favor of the upgrade as one may first think.
Frankly, I never buy gear based on upgradability. The market and technology is so fluid that while the one company is upgrading another has found a breakthrough technology which leaves the former in the dust.
Then, finally, even with upgrade, the unit may be easily bested by a competitor. Upgradability is no guarantee that one has the absolute best performance one can. It does assure that the march of technology does not completely pass you by, which is a good thing.
