As far as I know, the 603e is a BGA package, so with the right equipment, taking it out and putting a faster one in is fairly straightforward (without the right equipment, don't even attempt it). After that, you basically need to change the multipliers and clock crystals accordingly. IIRC, the multipliers are set via a block of four SMD resistors that you'd need to rearrange.
As tempting as it is, I've resisted the urge to fiddle with my BlizzardPPC. They aren't cheap to replace.