Changeling is a static ability. A card with changeling is every creature type, as its reminder text says. That means a Changeling creature is simultaneously an Elf, a Goblin, a Soldier, a Wizard, and every other creature type. It doesn't gain other card types or supertypes — just creature types. If the card isn't a creature, changeling still makes it every creature type for effects that check that.
In play, this makes Changeling cards incredibly flexible. Any effect that cares about a creature type — a lord that pumps Elves, a spell that searches for Goblins, or an ability that counts Soldiers — sees the Changeling card and includes it. Tribal decks can slot it into any tribe, and effects that let you choose a creature type always find a match. It also lets a single card satisfy multiple type-based conditions at once.
