"Retro Pilot" wrote in message
...

how long has it been since airports have gone from 3-to-4 letter
identifiers?


They haven't, not really. The 4-letter identifier is the ICAO identifier.
They've been in use since probably some time in the fifties. They've only
come into common domestic use in recent years because of GPS databases and
the need to distinguish airports from navaids of the same identifier. The
3-letter airport identifiers are just as valid as ever.



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


US domestic RVSM, Reduced Vertical Separation Minimums, was implemented
January 20, 2005.