Mayo,
Some comments...
There is for OS X:
• Airport 3.0.4 for 10.1.5.
• Airport 3.1.1 for 10.2.6 (includes 5.1.1 firmware, but you won’t install this unless you have an Airport Extreme base station).
• Apple AirPort Firmware 4.0.8 for (only) the snow (dual-ethernet) base station.
• Apple AirPort Extreme 5.1.1 Base Station firmware (included with AirPort 3.1.1).
If you are running OS X, you could install the updates as befits your Airport base station (Platinum, Snow, or Extreme), and your Airport will still work when you are running in OS 9. But if you don’t have OS X, I am uncertain how these updaters would be installed.
But if indeed you do have OS X, then yes, the updaters are seamless with no reconfiguring necessary. At least none was required on my system. But to be on the safe side, it is (still) best to take a screen shot of your Airport settings; and to read the ReadMe before the actual installation.
FWIW, I am now running, with never a problem, 3.1.1 on an original Airport that I have now had for ~3.5 years. I don't recall all the Airport updaters I have gone through, but I am sure there have been at least two, and both times they were no-brainers.
But if you do not have OS X, I think you are stuck with Airport 2.0.4.
HTH
Harv