This thread deserves a chance.
I make my own DACs using various kit parts, so I do not really replace DACs per se, so much as adding upgrades. I build DACs based on the Twisted Pear Audio modules (currently B-IIIse/Legato2 based) and the biggest upgrade so far was going to Salas Shunt regulators for the two digital pre-regulators, and the bi-polar supply for the analog output stage.
I think a lot of people make lateral moves in DACs, often to get additional features (like DSD). Then there are those who might make a real upgrade, say from a Auralic Vega to a dCS stack, etc.