Supposing you had a ghost or other incorporeal version of a creature that can change forms, such as an appropriately leveled druid or transmutation wizard (or any caster who can cast a spell like alter self or polymorph), would it be possible for them to temporarily gain a physical form through the use of said spells? Or would they simply become an incorporeal version of whatever they turned into?
It seems to me to make sense that they could become temporarily corporeal in this fashion, since there is already plenty of precedent for magic temporarily creating or destroying matter (and indeed it does so during many of these spells, as people regularly change size or assume forms with very little in common with their normal form), and there are a number of class abilities that allow you to temporarily go the other direction, such as Spirit Walk (Bones Oracle) and Incorporeal Form (Undead Bloodline Sorcerer), but I'd like to hear others' views...
Barring some specific rules to the contrary, you'd be an incorporeal XYZ (new shape). The new rules from polymorph don't actually turn you into the creature unlike 3.5, you'd still be a ghost/whatever you were that makes you incorporeal, just in a different shape.