All of the references have been sitting in a drawer, collecting dust, until today. I has been a few months since the 1000 hour tests were completed. I let the references and the DVM warm up for at least 2 hours before I took any measurements. At this point in time the DVM is expected to be within 155ppm @ 100mV, and within 80-90ppm for the 1V and 10V ranges.

The Voltage Reference using Digikey parts:

2020-02-20 2hrs warmup:

Set Point: = 0.6278V

Value          Range     Change
100.0166    100mV    (+101ppm)
1.000036     1V         (+13ppm)
2.50013      10V        (+80ppm)
4.99932     10V        (+20ppm)
10.00005   10V        (-2ppm)

I also calculated the change, in ppm, from the last readings taken at the 1000 hour point. The big disappointment was the 2.5V reference and the 100mV reference, which is derived from the 2.5V. The reading is outside the allowed range of the DVM. 80% of the change appears to be due to a big shift in the 2.5V reference voltage.

There are some differences: I believe that the 2.5V reference is a band-gap reference, where the other two references are derived from buried Zener diodes; the 2.5V reference is the only reference that is soldered to the PCB -- the other two are plastic DIP packages that are socketed.

The 10V reference is truly stellar with only a -2ppm drift.

The Voltage Reference using Chinese parts:

2020-02-20 after 2 hrs warmup:

Value              Range      Change
100.0043mV   100mV    (+118ppm)
1.000020V       1V          (+42ppm)
2.50021V        10V         (+92ppm)
10.00017V       10V          (-4ppm)

Again, the big loser is the TI 2.5V reference (both of the TI reference ICs were sourced by Digikey); and the winner is the LT1021-10. The amount of change in the 2.5V reference is nearly the same percent (and direction) between the two PCBs. 

Current Reference:

9.99935mA (-13ppm change)

0.999983mA (-11ppm change)

100.002µA (-12ppm change)

The current reference is based upon the LT1021-10 reference, so it is not surprising that the differences are quite small.

Conclusions:

Stick with the buried Zener reference and don't solder them into the PCB.

[Edit 2020-02-21: Added the following.]

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The day after getting the above results I took more measurements and got essentially the same results. But the 2.5V reference was not changing with the application of power, so I'm thinking it might be worthwhile to just recalibrate the 100mV reference and continue onward, which is what I did. Here's the new readings to compare with future data:

Digikey Box:

99.99952mV (averaged)

1.000027V

2.49975V

4.99935V

10.00005V

AliExpress Box:

100.00077mV (averaged)

1.000009V

2.50012V

10.00019V

I discovered that the DVM will calculate mean and SD from the readings. It is nigh impossible to get consistent readings from the 100mV source. Small temperature variations cause galvanic errors and then there is noise. Expect ±2µV uncertainty.

The reference boxes are going back on the shelf for a few months. We'll take them out and measure again near the first anniversary of the DVM.