In actuality, the upgrade is not so much an upgrade as it is a down price. That is, you pick the cabin you want, and you pay the price of the cabin one level below.
so if you want a C1, you pay for a C2. Or you book an A1 but pay the cost of an A2.
now, in order for this to take place, there must be an available cabin in the lower category so they can get the price it would cost.
this is available for insides, outsides, verandahs, concierge and Aqua rooms, but not suites.
and this perk is not limited to Zeniths