Dachhocker

If an effect causes a creature with changeling to become a new creature type, it will be only that new creature type (unless the effect says "in addition" or similar). It will still have changeling; the effect making it all creature types will simply be overwritten.

If an effect causes a creature with changeling to lose all abilities, it will remain all creature types, even though it will no longer have changeling. This is because changeling applies before the effect that removes it.

The subtype Shapeshifter that appears on the type line is mostly there to reinforce the flavor. A creature card with changeling is just as much a Kithkin, a Goblin, a Merfolk, and a Brushwagg as it is a Shapeshifter.

You don't have to choose any targets for Rooftop Percher's triggered ability. However, if you do and all of the targets are illegal as the ability tries to resolve, it won't resolve and none of its effects will happen. You won't gain life.

Changeling is a characteristic-defining ability. It functions in all zones, not only while a card that has it is on the battlefield.