Scientologist here,
You do seem to have some misunderstanding.
Not sure where you are getting your information from exactly, but let me define a couple of words that you are using. This should help to answer your question.
THETAN: 1. A noun created from the Greek letter "theta" a mathmatical symbol used in Scn to indicate the source of life and life itself.
2. The awareness of awareness unit, the being who is the individual and who handles and resides in the body.
3. the person himself- not his body or his name, the physical universe, or his mind . The identity which is the individual.
There are more specfic definitions but let's keep it simple.
Basically YOU are your own immortal soul.
The word THETAN was coined to avoid confusion as words like spirit, or soul have become confused by definition.
CLEAR 1. A thetan who can be at cause knowingly and at will over mental matter , energy, space and time for himself only.
2. A Clear possess attributes, fundemental and inherent but not always available in an uncleared state. These attributes have not been suspected of man and are not included in past discussions of his abilities and behaviour.
Again there are more definitions but I am keeping it simple.
To answer your question, once a Thetan has achieved the State of Clear he has regained this state as a spiritual being, for himself.
There are higher states above this. They are all in the direction of
putting the individual more and more at cause over life and the various dynamics of life including the physical universe.
What future existences the thetan, being, or YOU may have, becomes your choice. But if you choose to return , (pick up another physical body) it will be causatively. In other words it will not be because you are trapped in the endless cycle of birth, life and death but can operate with or without a physical body.
Since Scientology has only been available for the past 55 years, these upper level states are not all fully released as yet. It is hard to say what individual choices will be made. We have to get a lot more people to achieve the State of Clear first.
By the way there are already many thousands who have achieved this state and some of the higher Levels, myself included.
Hope this helps some what. Let me know.