http://support.apple.com/kb/HT1395 explains which Macs may have difficulty upgrading to 10.2 or later without the proper firmware installed first. This is part of a series of tips for updating PowerPC Macs to 10.2, 10.3, 10.4 and 10.5.
How to find the download links for these firmware updates are mentioned on this tip.
Note: Mac OS X versions beta, 10.0.0, 10.0.1, 10.0.2, 10.0.3, 10.0.4, 10.1, 10.1.1, 10.1.2, 10.1.3, 10.1.4, and 10.1.5 are not covered by my series of tips. Very few Mac OS X titles were only developed for these operating systems. All Macs that support Mac OS X, support a minimum of 10.2 as long as the firmware is updated, except those Macs that are newer than its release on August 24, 2002. Retail releases of December 19, 2002 of 10.2.3, also supported Macs that were older. Retail discs of 10.2 and 10.2.3 had this label, and did not say Upgrade or Dropin or OEM on them. System specific discs for 10.2 were required for newer Macs that needed to run 10.2, that were older than 10.3's release of October 24, 2003. 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