I don't know Spheres of Power. A player of mine brought it up to me in a prior campaign and I declined to learn the system. However, being familiar with the ZG campaign and its associated rules, here's my (very unofficial opinion):
ZG has a few house rules with regard to how magic works. You can find these in the Player's guide with regard to flying, teleportation, and planar magic. These broad concepts should be easy enough to port into any magic system. ZG gets a whole raft of additional house rules at the beginning of adventure 10 (and these are previewed in a restricted capacity in adventure 7), and these have a bit more of a driilldown into how specific spells work and might require more of a conversion into an alternate magic system. If you plan on playing that far.
ZG uses quite a few non-standard monsters and rules for its NPCs. On one hand, this will make "converting" to an alternate magic system easier, simply because there are so few NPCs who were using the core magic system to begin with. They will just remain as the unique rules-hack snowflakes that they are, and need not concern themselves with what rules govern the PCs. On the other hand, depending on how Spheres of Power is implemented, this could be a real headache. For example, I know that 3.0 Psionics kindof requires the PCs to go up against psionic NPCs pretty frequently, or else the whole system breaks down. If Spheres of Power is like that, you're in a lot of trouble. But if Spheres of Power is just a way for PCs to cast stuff, and doesn't make any assumptions of the spellcasting ability of their opponents, then things will be quite easy for you.
If you're worried that there's any rules-specific minutia that ends up being super-plot relevant, then no, you're in the clear. This isn't like Rise of the Runelords, where the specific specialist and opposition schools ends up having a massive impact on the plotline in the second half, and if you try to convert the rules away, you break down core setting assumptions. Just the broad strokes around gold and planes (which *are* plot-critical houserules).
Anyway, dunno if that was helpful, but I hope it was. And ZG is the best pre-written campaign I've ever come across. (It was difficult for me to dethrone GPC in that regard.) But this campaign hard to run, and requires a ton of prepwork. If you put in the energy, the thing really sings. Enjoy!