AV-1 is Northrop's airframe, I do not know of any upgrade for that bird.
AV-2 thru AV-6 were ungraded to production version, about that time.

Not true. The final B-2 production run was to be 20 planes after the program
was cut from 132 then to 75 and finally 20. AV--2 thru -6 were to be flight
test assets but when the cut to 20 came, they were included as part of the 20
iin SAC's Bomber Roadmap with plans to upgrade them to final production
configuration. AV-1 was so different than the others that it was warranted to
not be worth the cost of upgrade. At that time Northrop-Grumman was quoting
$350 MIllion. Then the $550M long lead initiative came along. the AF took
the position that It couldn't afford to support another 20 and the $550M got
diverted to upgrading AV-1 to operational configuration. Interestingly, the
cost for the upgrade rose from $350M to $550M at the same time. It was the
final B-2 out of 21 delivered. It is flying at Whiteman today.

2001(IIRC), the non flying structural test article was recently delivered to
the AF Museum.
Pilot shortages were not the issue with a 40 plane fleet. At 150+
maintenanace manhours per flight hour, there weren't enough greensuiters at the
time to maintain a larger fleet. I trust that number has gone down some.