Having just joined the 'new A3' club I can pass along my experience.
I purchased a late 1990s vintage 3458 for around $2k. Advertised as passing self test and auto cal which it did indeed do when it arrived. The first thing I did was:
Monitored it against my relatively stable 731B
Started doing a daily CAL?72 check
Sure enough, the CAL72 value was drifting down at a fairly steady rate of around .75ppm per day. In addition, the 731B voltage was drifting up about the same rate. Performing an ACAL would immediately drop the measured voltage down to the expected value. Thus, I was fairly sure I had the dreaded drifty A3 disease.
So I contacted Keysight and arranged an A3 exchange. The initial cost is $1486 but they supposedly refund $286 if you return the old board. Good news, free shipping
The board was backordered for about 3 weeks, during which I continued to monitor the CAL72 value. The drift seem to slow a bit and sometimes time flattened out but still, over 3 weeks, was out of spec with regards to the service note.
My new A3 arrived a couple of days ago and was about a 20 minute swap out. The first couple of days of CAL72 numbers are now down in the .05ppm/day range. In addition, measuring both my 731B and 5440B confirm the stability of both of the those sources and the 3458. So far, I'm a very happy with the replacement.
I haven't yet sent back the old A3. In fact, there was no prepaid label or other instruction on how to do so. Supposedly, I have 30 days to figure it out.
I would really love to know what causes these A3 units to drift. Mine was drifting pretty darn linearly regardless of whether it was powered on or not.