[JFreak]

PT 6.4.1 for MIX and HD should authorise with same serial number, but uninstall/reinstall is probably needed because of different drivers. I do not really know because I always install from empty hard drive, but I have a hunch.

First of all make sure you have that serial available before you do anything, because once you start messing around and find you don't have that serial you cannot even revert back to your current setup. You can of course try with simply swapping the cards and see if it works.

BUT... and this is a very big but...

G4 can only take the old style PCI cards which means they top at PT version 8.0 (if your computer can run OSX 10.5 Leopard) so if you are planning to upgrade, buy such a (latest generation) PowerMac G5 or Intel Mac Pro that accepts PCI-express cards. With those cards you can go up to latest PT10 before Avid switched to 64-bit only.

If you do that, and I mean upgrading from 6.4.1 to PT8 or PT10, you need to update *all* plugins as well. Those old plugin installers are becoming hard to find, so bear that in mind when you decide what to do.

Interfaces you do not need to change, they are good even with the current PT version.