A number of DACs/streamers/combination DAC-streamers are physically modular, i.e. are upgradable. I tend to favor these. Sonnet, Bricasti, totaldac, aqua, MSB come to mind. So you don't have to dump/sell the box, if you want to update them.
FPGA DACs such as dCS and PS Audio can receive firmware updates over time, which is also a nice feature.