Well, assuming that IM can decided to accommodate an upgrade request from a GS member even though there may already be others on the waitlist with confirmed instruments, they basically have this special GS "inventory" already. It may be that IM creates it on the spot when looking at a GS request (and not something which is maintained like other inventories), but in any event it is certainly not something which should be transparent to vast numbers of MP members like NY inventory is.