Sure, you've got space to add 3 or 4 hard drives to your MDD G4, or you could replace the existing HD, or you could add the old HD (jumpers to slave on the added HD), Your system SHOULD boot up with the older HD just fine. One problem I can forsee, will your new G4 be an OS 9 bootable system, or is that any concern for you? If this were my system, I would add the old HD as a slave, boot up to the old HD, wipe the new HD, clone the old HD to the new HD, boot to the new HD, and reload the 10.2.6 combo update on your new HD. Then do normal maintenance scripts, permission repairs, etc. Should give you a great running system. After that you should wipe the old HD, and use it for storage.