The Ever-Changing ’Dane copies the printed values of the creature plus any copy effects that have been applied to it. It won’t copy any other effects that have changed that creature’s power, toughness, color, and so on. The Ever-Changing ’Dane won’t copy any counters on the creature, but The Ever-Changing ’Dane will retain any counters it already had on it.

If The Ever-Changing ’Dane becomes a copy of a token creature, it copies the original characteristics of that token as defined by the effect that put it onto the battlefield. It won’t become a token creature.

The copy effect lasts indefinitely. Often, it will last until it is overwritten by another copy effect (if it copies another creature on a future turn, perhaps).

When The Ever-Changing ’Dane becomes a copy of a creature, it’s neither entering nor leaving the battlefield. Any enters-the-battlefield or leaves-the-battlefield abilities won’t trigger.

If another creature becomes a copy of The Ever-Changing ’Dane, it will become a copy of whatever The Ever-Changing ’Dane is currently copying (if anything), plus it will have the activated ability.