Changeling was introduced in Lorwyn, this is on creatures that are, in the game, have all creature types. But that's inconvenient for printing purposes, so this keyword is used instead.

702.70. Changeling

• 702.70a. Changeling is a characteristic-defining ability. "Changeling" means "This object is every creature type." This ability works everywhere, even outside the game. See rule 405.2.
• 702.70b. Multiple instances of changeling on the same object are redundant.

Rules Translation

• 702.70a. - This ability isn't static, it is always happening in all zones, even out of play. It defines the characteristic.
• 702.70b. - Changeling does not stack.