Originally Posted by
tibbee
I'd suggest upgrading your weapon (at least assuming you're not using an LFR weapon or something) even if it will be replaced, because you gain roughly 550 Spell Power on top of roughly +15 to every stat on it. In comparison, if you instead upgraded something like bracers or a cloak, you would only get +30 to each stat.
Sure, you might end up getting a better weapon eventually and having to upgrade it wasting the VP, but when you're trying to push progression, whatever increases your statistical player power the most is what helps you the most.