Metamorphosis

Metamorphosis is an illusion, invented by John Nevil Maskelyne, but made famous by escape artist Harry Houdini,  in which an assistant (Houdini employed his wife Bess) is locked inside a large box or trunk, often after being restrained with handcuffs, ropes, bags, etc. The magician stands on the trunk and holds a curtain up to momentarily conceal his entire body. 

John Nevil Maskelyne

When the curtain is lowered, it is now revealed to be the assistant standing atop the box, the magician and assistant having changed places instantaneously. 

When the box is opened, it is shown to contain the magician, restrained as the assistant had been. In some acts, the magician is locked in the box first, then appears standing atop the box.
 

The Pendragons are a husband and wife team of magicians from America who perform this old trick, but with a new spin; they do it ten times faster. They are well known for their fast and dangerous illusions. 

Their metamorphosis illusion is regarded as the fastest trick ever to be performed in history, exacting the illusion in under a second.  The illusion won them a place in the 50th edition of The Guinness Book of Records, under the heading of the "Fastest Transformation Illusion"

The Pendragons

Variations include the Aquarian Illusion performed by Mark Wilson, wherein a tank filled with water is used and a beautiful swimsuit-clad woman assistant is locked inside underwater, and Penn and Teller performing the illusion in an underwater theater with the female assistant handcuffed and locked in a steel cage on the bottom. 

These variations add an element of peril. If something were to go wrong, the assistant (or magician) could possibly drown.

Here are the amazing Pendragons performing their metamorphosis in which the switch is made literally in the blink of an eye: