MTG Salvation

If a basic land becomes a creature, is its creature type its name (for example, plains, island, etc.) If this is correct, does a separate creature with "changeling" (like avian changeling) share that as a creature type (plains, island, etc).

How did the land become a creature? Creature types and land types are different. Swamp is never a creature type so a Changeling is never a Swamp. Normally, anything animating a land will give it a creature type (see Life and Limb or Raging Ravine). If it doesn't grant a creature type, it doesn't have one. Vehicles are the best examples of this. When they turn into creatures, they do not have a creature type (they are still Vehicles as that is an Artifact type, but you can't name "Vehicle" with Metallic Mimic)

animate land would be an example of turning a land into a creature, but I believe you explained it well. If I used that card, my land would not have a creature subtype (though it would still have its land subtype). Thank you.