Instrument upgrades using BXP1s, SWUs and mileage are prioritized over all elite complimentary upgrades when the flight goes to airport control and the waitlist is created. Of course, CKs with an instrument upgrade will hold priority over an ExPlat with one.

But yes that means you could get your PPRO upgrade days before the flight while those with instrument upgrades are still waiting for "C" inventory to clear. The algorithm behind how this is done is closely held, though, so there's no way to really know how the system determines when to open X inventory (domestic comp upgrades) and/or C inventory (instrument upgrades) when the elite windows begin to open.