I’ve lucid dreamed of being a bouncy ball, a point of light, an expanding star, and a floating eye, and I’ve had many entirely bodiless lucid dreams. [...] On rare occasion, we can even find ourselves inhabiting two different dream bodies simultaneously.
- Excerpt from Llewellyn’s Complete Book of Lucid Dreaming by Clare R. Johnson, PhD. PDF found here.It can be hard to change one's human body in a lucid dream. Some LDers find that their LD body won't forget their IRL body, leading to dissonant situations such as looking different in a mirror but feeling their old body's limbs. The brain may not be used to the sensations it would have to give the dreamer from the new body.
However, there are LDers who find it easier to change into something inhuman because it's such an extremely different body that their brain finds it hard to slip up into a human one. In addition, many dreamers (lucid or not) can be inhabiting a different body
from the start of their dream, no effort for changing needed.
The methods for fully changing one's body depend on the person. More of those methods:
Believe that the new body is the true body that belongs to you. It can then feel natural to change.Concentrate on how your body changes into the lucid dream one. For example, imagining the feel of fur sprouting from your arms if you wanted to become a werewolf.(If the body being changed into is one of a living creature) Behaving like how the desired body behaves. For inhuman bodies, it would be acting out what comes naturally to that body regardless of whether a human body can do it or not - ignore any human-like quality the current body has and your brain may forget your humanity as well. For human bodies, it can be as simple as doing activities that you would do in that body.(If the body being changed into is one of a non-living thing) Imagine your five senses slowly fading. For sight, it helps to remember the darkness when closing your eyes. For smell, the lack of scents when holding your breath. For touch, the lack of physical sensations when first waking up. For sound, any quiet moment. For taste, any time when you haven't had something in your mouth.Ruminate in the waking world about having a different body and you may find yourself in it from the get-go of the dream, similar to ruminating on having lucid dreams to get a lucid dream. Ruminating on details such as what you imagine that body to feel like could help make the experience 'realistic'.Make or find something in the dream that would change your body for you. One example is of a dream character with mystical power, who may grant you your new body if asked. Another is to find a machine, such as passing through a metal-detector-like structure and emerging through it with your new body.Related to the previous bullet, it can be enough to think, "My body will change to what I want once I [do x action] in a dream." That action can anything, including walking through a door, spinning in place, looking in a mirror, saying a magic word, and drinking a potion.Many of these tips are meant to induce a degree of dissociation. If this is not wanted, the last 2 bullet points are ways to change the body without needing much dissociation in the process of obtaining the body.
There are no limits to lucid dreams. "The sky's the limit," doesn't apply here either, since one can change their body into the sky. Changing one's body in a lucid dream will never be impossible.