What the upgrade changes
The base card has both Ethereal and Exhaust. Ethereal removes it from combat if it is still in hand at turn end; playing it also exhausts it. Leaving it unused does not save it for the next shuffle.
Upgrading removes Ethereal, but not Exhaust. That gives an unused copy a chance to cycle back later; it does not make a played Apparition reusable. The upgrade also does not grant Retain. Without a separate retention effect, an unused upgraded copy is discarded normally rather than remaining in the next hand.
Intangible reduces each damage or HP-loss instance to one for the turn. It is not the same protection as the single Buffer from Lucky Tonic. Several enemy hits can still cost multiple HP, and the effect does not restore health. Account for the full sequence when surviving at very low HP.
Play an unupgraded Apparition when its protection prevents a dangerous turn rather than waiting for an ideal later draw that may never occur. If the deck needs to save it for a particular boss window, evaluate the upgrade and access plan together with opening-hand consistency.