http://support.apple.com/kb/HT1395 explains which Macs may have difficulty upgrading to 10.2 or later without the proper firmware installed first. How to find the download links for these firmware updates are mentioned on this tip. Having the Mac OS 9.2.1 installer CD you would have the firmware updates already. Of course the first thing you want to do before any of this begins is to backup your data. If you do not install the correct firmware the internal display of various iMacs may no longer work, and for some of those iMacs that's the only display they can have. Note, the firmware update in many cases is only available if you first install Mac OS 9, or boot off the Mac OS 9 installer CD that is compatible with that Mac. Be sure to read: http://support.apple.com/kb/HT2191 to determine which Mac OS 9 can be installed first, and then install the retail version newer than what shipped with the Mac. Use the guides below to compare with the HT2191 to accurately determine which 9 may be needed. Note: Mac OS 9 retail shipped in the sequence of versions 9.0, 9.0.4, 9.1, 9.2.1, and has an orange 9 logo on a white CD, not saying Update, Dropin, or OEM.
http://support.apple.com/kb/HT1362 - how to identify your iMac
http://support.apple.com/kb/HT3065 - how to identify your Powerbook G4
- note the only Powerbook G3 that needed a firmware update has a firewire port, which looks like this on the rear panel:
Sometimes 'C' key booting will fail with these 9 CDs, and instead the startup manager described in this article lets you boot the 9 CD.
However, instead of selecting the hardware test, you select the 9 CD icon that shows up. Lastly if a Mac was formerly formatted for Mac OS X, it may not be able to have 9 bootable installed without first erasing and reformatting the Mac, following these instructions: http://support.apple.com/kb/TA26834
Naturally you must backup your data first before erasing the machine, if you have any hope of recovery. Once you have upgraded the firmware, see if your Mac is compatible with 10.3 or later, as this tip gives you your possible upgrade paths: https://discussions.apple.com/docs/DOC-2139