I just yesterday installed Final Cut Studio 2 onto my G5 Quad. Having read the quagmire of misinformation about serializing FCS2 from a previous purchase, I cloned and initialized the boot drive with MacOs 10.4.9 and Quicktime 7.1.6 and then installed FCS2 from discs.
Upon starting the install it first asked for the serial number from the ($499) FCS2 box and then a dialog came up asking for a "previous" serial number. After hearing that you would need to use the original serial number of the copy of FCS or FCP that you "originally" purchased, I said" This is an upgrade from FCS1 and that is the serial number that should work". So, I put it in and IT DID work!
No mucking around with previously previous serial numbers. I had bought FCP 3 and then upgraded from 3 to 4 to FCS1 but as in all other upgrades it only asked for the last serial number from the last upgrade.
FYI!