I am looking for a word that can be used to describe a person who can switch bodies with another human

I'm going out on a limb here. I would suggest, as you seem inclined, using the diminutive suffix -ling with the appropriate descriptive word indicating an immature one, new one, or small one. The limitation is with the descriptive word that you use. From your description you describe a mutual experience - to switch, swap, trade, or exchange awareness. I think "switchling" might work. Or, if the switching process allows, possibly "graftling" or "spliceling".

http://www.dictionary.com/browse/-ling

transmute TFD

trans·mut·ed, trans·mut·ing, trans·mutes

To change from one form, nature, substance, or state into another; transform

As in:

This young woman has known since she was a child that she was able to transmute into another body for an unknown amount of time but she could not control it.

You may need to come up with your own SciFi neologism. This site's answers does not participate in such ... though anyone can do so in comments. Check the synonyms of this word and any others in answers and comments. Check out this article body-swapping for ideas. Good luck.

Good luck!

Assuming that you mean the character can’t control the ‘ability’, rather than can’t control the body they occupy, then you seem to be talking about a form of possession. Most commonly this is identified as a supernatural phenomenon taking the form of demonic or spiritual possession

Demonic possession, psychokinetic control of a person by the Devil or other malevolent spirit.

Spirit possession, psychokinetic control of the behavior of a living thing or natural object by a spiritual being

but essentially comes from the obsolete meaning of ‘possess’ as ‘to inhabit or occupy’ giving a modern meaning of

To take control of someone's body or mind, especially in a supernatural manner.

As to what people with the ability to possess others might be called, it is quite likely that the general population might call them witches or devils, what they call themselves may be very dependent on how they see the ability and how they use it.

Terry Pratchett used the gentler concept of ‘borrowing’ for characters who could ride in the mind of another but chose not to control the owner (other than the odd nudge).

This is typically called a ‘walk-in’. A walk-in is a soul that moves into another body, typically when the original soul leaves or dies, or sometimes they (choose to) co-habit the one body.

I have a friend whose baby died as a foetus, and that soul ended up staying and co-habiting with her. She learned a lot of new spiritual wisdom from that walk-in soul.

https://en.m.wikipedia.org/wiki/Walk-in