In a previous article, "Retro Pilot" said:
how long has it been since airports have gone from 3-to-4 letter
identifiers?

You're confusing two types of identifiers. FAA identifiers are three
letters or letters and numbers. ICAO identifiers are four letters long,
the first letter identifies a region, and they don't allow numbers. The
continental US is the region with the identifier "K". By convention,
those airports in the continental US with ICAO identifiers just use their
FAA id with a K put on the front for their ICAO id. Most of the world
uses a more complicated system with the second letter designating a
sub-region. Hawaii, Alaska, the US Virgin Islands and other places use
"P" and a second code letter, and then the mapping between the FAA id and
the ICAO id is more complicated.

US pilots became more aware of the ICAO identifiers when the US started
using METAR and TAF weather reporting formats, the same as the rest of the
world. That happened about 10 years ago. We had to change the FAA id of
several airports to elminate numbers so that they could be given ICAO ids.
Also, GPSes use the ICAO id rather than the FAA id when the airport has an
ICAO id, just to reduce confusion between airport ids and co-located or
nearly co-located VORs with the same FAA id.

also, when did ATC go from 2000 feet vertical separation above FL290 to 1000
feet?

RVSM (Reduced Vertical Separation Minimums(?)) were phased in quite recently.
They came about because of improvements in altimeter technology, and maybe
also as a result of TCAS and mode-S.


--
Paul Tomblin http://xcski.com/blogs/pt/
please excuse my typing, but my whole left arm is in a cast. and i don't
mean _the king and i_.